Astronomy & Science

This Week’s Sky at a Glance, December 5 – 13

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Some daily sky sights among the ever-changing Moon, planets, and stars.WEBvic14_Dec5-6ev

The bright Moon shines near Aldebaran on Friday the 5th and about equidistant from Aldebaran and Betelgeuse on Saturday the 6th. For clarity, the Moon is shown three times actual size. It's positioned exactly for an observer in the middle of North America. (The blue 10° scale is about the size of your fist at arm's length.)

Friday, December 5

The Moon is essentially full this evening and Saturday evening both (it's exactly full at 7:27 a.m. Saturday morning EST.) On Friday evening in the Americas, look for Aldebaran less than about 2° from the Moon. Watch the Moon shift east with respect to it during the night.

Saturday, December 6

By mid-evening the Moon shines high in the east. It's in a starry part of the sky. Aldebaran is now to its upper right. To the Moon's lower right is Aldebaran-colored Betelgeuse. Much farther lower left of the Moon are Castor and Pollux. And high to the Moon's upper left? There's Capella.

Sunday, December 7

Look right of the Moon this evening for Betelgeuse, and farther right for the rest of Orion. Watch far down below the Moon in midevening for Procyon to rise. Then watch equally far below Orion's Belt for brighter Sirius to rise a little later (as seen from the world's mid-northern latitudes).

Monday, December 8

The waning Moon rises by 7 or 7:30. As it climbs higher, look to its left for Pollux and, above Pollux, Castor. To the Moon's lower right, Procyon is on the rise.

Tuesday, December 9

The waning gibbous Moon, now in Cancer, is well up in the east by about 9 p.m. depending on your location. Look to its right for Procyon. Farther to the Moon's upper left are Pollux, and above Pollux, Castor.

Wednesday, December 10

Mid-December is when the dim Little Dipper hangs straight down from Polaris around 9 p.m.

Moon passing under Jupiter and Regulus in early dawn, Dec. 11 - 13, 2014.

Early risers can watch the Moon pass below Jupiter and Leo on the mornings of the 11th, 12th, and 13th.

Thursday, December 11

The waning Moon rises in the east by about 10 or 10:30 p.m. tonight, with bright Jupiter shining to its upper left. Fainter Regulus, to their lower left, forms a nearly equilateral triangle with them. The scene at right shows how you'll see them by early dawn Friday morning.

Keep an eye out for early-arriving Geminid meteors! This annual shower should peak late on Saturday and Sunday nights; see below.

Friday, December 12

This is the time of year when, around 8 or 9 p.m., Cassiopeia crosses very high in the north as a flattened letter M. When do you see it lined up perfectly level?

Saturday, December 13

The Geminid meteor shower should be at its strongest late tonight and tomorrow night. Bundle up even more warmly than you think you'll need, find a dark, shadowed site with an open view overhead, lie back in a reclining lawn chair, and watch the stars. Be patient. Under a fairly dark sky you may see a meteor every minute or two.

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Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby; for an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

Pocket Sky Atlas

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's still less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability, which means fairly heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet Roundup

Mercury is hidden in the glare of the Sun.

Venus (magnitude –3.9) is beginning to peek through the glow of sunset. Look for it just above the southwest horizon about 20 minutes after sundown. Binoculars help, and the farther south you live the better. Venus is beginning a long, slow evening apparition that will last into next summer.

Mars (magnitude +1.0, in Capricornus) still glows in the southwest during and after twilight. And it still sets around 8 p.m. local time.

Jupiter (magnitude –2.3, in western Leo) rises in the east-northeast around 10 p.m. About 40 minutes later, fainter Regulus (magnitude +1.4) rises below it. By dawn they shine high in the south, with Regulus now to Jupiter's left.

Saturn (magnitude +0.5, in Libra) is emerging into the dawn sky. In early dawn, look for it low in the east-southeast, far below Arcturus and Spica. Binoculars help.

Uranus (magnitude 5.8, in Pisces) and Neptune (magnitude 7.9, in Aquarius) are still well up in the southern sky right after dark. Use binoculars or a small telescope and our finder charts for Uranus and Neptune.

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All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 hours.

"Science is built up of facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house."

— Henri Poincaré (1854–1912)

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Warm Up with December’s Geminid Meteors

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The annual Geminid meteor shower, one of the best shooting-star displays each year, returns to our skies late this week. Despite interference from moonlight, plenty of bright meteors should still shine through.

Bright Geminid meteor

During the 2004 Geminid meteor shower, Alan Dyer caught a bright fireball with a tripod-mounted digital camera. He used a wide-field, 16-mm lens for a 1-minute exposure at f/2.8 with an ISO setting of 800. Expect to shoot a lot of frames before you get this lucky.

When it comes to annual meteor showers, mid-December's Geminids rank right up there with August's Perseids as a great, dependable display. Dynamicists predict that this month the Geminids will peak on the nights of Saturday and Sunday, December 13th and 14th.

On those nights, under a clear, dark sky, you might see a "shooting star" every minute from about 10 p.m. local time until dawn. If your sky has lots of light pollution then the count will be lower, but the brightest meteors will still shine through. Light from a last-quarter Moon won't become a hindrance until it rises close to midnight.

The Geminids' peak might still be several days away, but if you've got a cloud-free night between now and then, start looking! This shower offers quite a long run-up to its peak. In fact, cameras in NASA's All-Sky Fireball Network started sweeping up bright Geminids back on December 1st.

So why do we see these particular meteors every mid-December, like clockwork? Meteor showers happen when our planet plows though a stream of fine particles that have been shed by a comet and spread out along its orbit. Earth crosses the Geminids' orbit each December. But these meteors are unusual — their source isn't a comet but an asteroid, called 3200 Phaethon. Phaethon isn't very large, only about 3 miles across, and it was discovered fairly recently, in 1983. Before then no one knew where the Geminids came from.

Phaethon's most remarkable distinction is that it approaches the Sun closer than any other named asteroid: its perihelion is only 13 million miles from the Sun. That's less than half of Mercury's perihelion distance, and it means temperatures can climb to more than 1,000° Fahrenheit. Some astronomers think Phaethon once was a comet, whose ices were baked away long ago. Others think it might be a "rock comet", whose surface is cracking and crumbling under the Sun's intense heat.

So the Geminid meteors are caused by comet crumbs, or asteroid crumbs, or maybe both. But no matter what their true source, they provide a great visual treat this time of year.

Track a year's worth of sunsets and sunrises — and all the celestial highlights that happen in between — with our annual Skygazer's Almanac (for latitude 40° north).

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Flying Through Cosmic Voids

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In the February 2015 issue of Sky & Telescope, author Marcus Woo walks readers through the science of...nothing.

Empty space makes up most of the universe by volume, and though most astronomers are drawn to light, it's in these blackest of voids where we can potentially learn the most about dark energy, dark matter, and the growth of galaxies.

The sheer vastness of cosmic voids makes them difficult to comprehend. But animations and simulations can help the human mind conceive of expanses several hundred million light-years across, even if the only we can understand such emptiness is by focusing on its populated edges. Fly through the universe — theoretical and observed — in the simulations and animations below.

The 400,000 galaxies shown in this animation based on the Sloan Digital Sky Survey provide some perspective: fly through clusters and walls of galaxies, and the voids that surround them.

Large-scale simulations, such as the Bolshoi and Illustris simulations, are ultimately designed to compare results with galaxy surveys such as SDSS. The Illustris simulation shown below demonstrates the evolution of a galaxy cluster, incorporating the effects of radiation (from gas-guzzling black holes and forming stars) on cosmic evolution. But keep an eye to the negative space and you'll witness the voids evolve as well.

Most simulations focus on galaxy and galaxy cluster evolution, but the rare astronomers who focus on the lack of light model cosmic voids, tracking their evolution over time. On a smaller scale than the above Illustris simulation, Erwin Platen (University of Groningen, The Netherlands) simulated the growth of a single void in a universe with standard cosmology. The simulation frames shown below demonstrate how matter flows toward the edge of the void over cosmological time, drawn away from the center under the force of gravity.

Erwin Platen

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Have Astronomers Discovered an Evicted Black Hole?

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Orphaned black hole or weird supernova? A mysterious source of radiation has left astronomers contemplating exotic explanations.

Galaxy collisions produce some of the best fireworks in the universe. Interstellar gas meets head-on and explodes into star formation, and even though stars themselves rarely collide during a galactic pile-up, their orbits are forever altered as the galaxies merge.

Dwarf galaxy and its ejected black hole?

The Keck II telescope in Mauna Kea, Hawaii, shows dwarf galaxy Mrk 177 and SDSS 1133, a mysterious source of radiation 2,600 light-years removed from the galaxy's center.
W. M. Keck Observatory / M. Koss & others

At the center of all this activity, two supermassive black holes, one from each galaxy, coalesce into one. In fact, to make the biggest black holes in the universe, such collisions might happen over and over again. And when they do, gravitational waves released in the process often give the merged black hole a kick, like a gun’s recoil. A particularly strong recoil might even evict the black hole from its host altogether.

Yet recoiling black holes have been surprisingly hard to find — only a few known candidates exist. So when Michael Koss (ETH Zurich, Switzerland, and University of Hawaii, Honolulu) and colleagues went in search of these rare beasts, they were delighted to find an odd source of radiation near Mrk 177, a dwarf galaxy some 90 million light-years away from Earth.

The galaxy’s smooshed shape could indicate a recent galactic pile-up. And the radiation source, formally known by its telephone number, SDSS J113323.97+550415.8, lies some 2,600 light-years from the dwarf’s center, a reasonable distance for an evicted black hole.

When Koss’s team crawled through the Digital Sky Survey archives, they found this object has been going strong since 1950, and even brightened briefly by 2.5 magnitudes in 2001 before settling down again. The team obtained a bucketload of additional observations, including visible-light, near-infrared, and X-ray imaging, as well as visible-light spectra.

The following video shows some of the archival imagery of SDSS J1133 taken through a variety of filters and instruments, starting in 1950 and peaking in brightness in 2001:

Many of these observations support the ejected black hole scenario. Both the variability and the object’s broad emission lines are typical of a gas-guzzling black hole. And based on the width of the broad H-alpha emission line, which corresponds to the speed of gas whizzing around the black hole (if that’s what it is), the authors estimate a mass of 1 million Suns.

But it’s not a shoo-in. Astronomers aren’t even certain how often dwarf galaxies can grow their own supermassive black holes — such a combination might be quite rare.

“On one hand, it would be quite surprising to expect to catch the aftermath of the merger of two dwarf galaxies, both of which had supermassive black holes,” says Laura Blecha (University of Maryland), one of the paper’s coauthors. But, she adds, even modest kicks of a couple hundred kilometers per second could push the merged black hole beyond the dwarf galaxy’s weak gravitational reach.

This video shows the Sloan Digital Sky Survey data used to track some of the object's past, as well as a simulation that shows how a dwarf-dwarf galaxy merger may have evicted the black hole:

“I think it's definitely the strongest candidate [recoiling supermassive black hole] we've identified so far,” Blecha says.

Black Hole or Supernova?

But the data conflict enough that even the team members disagree on what this object is. A luminous blue variable (LBV) star, like the famous Eta Carinae, could also display broad emission lines and variability, especially if it ultimately ended its life in a supernova.

eta-carinae-341px

The most famous example of a luminous blue variable (LBV) star is the primary star of Eta Carinae, which ejected 10 Suns' worth of mass between 1838 and 1845.
Nathan Smith / NASA

The LBV scenario provides a better explanation for some of the other observations that cannot easily be explained if the object were a black hole, such as the object’s narrow Fe II emission lines. And a supernova would also explain the object’s 2001 peak in brightness.

But an LBV-plus-supernova scenario isn’t a shoo-in either. To explain the decades of above-average brightness, the star would have had to experience the largest pre-supernova mass loss ever recorded, and even then, the broad emission lines remain difficult to explain.

Nevertheless, co-author Jon Mauerhan (University of Arizona) strongly supports this option. “This is the simplest explanation of the observational data, and one that is consistent with other objects we have recently observed.”

“The recoiling black hole is a very interesting plausibility that cannot be ruled out,” Mauerhan adds, “although this is a very exotic interpretation of the data for which there is not much precedence.”

Regardless of what it is, it’s clear that this object is rare.

“It's either a new and exotic type of object (a recoiling black hole) or the strangest, most exotic example of a very common type of object (a supernova),” Blecha says.

High-resolution radio and ultraviolet observations, or high-sensitivity X-ray observations could settle the debate once and for all — and soon. “We have good prospects for distinguishing between the two scenarios with new observations in the coming months and years,” Blecha says.

Reference:
Michael Koss et al. "SDSS1133: An Unusually Persistent Transient in a Nearby Dwarf Galaxy." Monthly Notices of the Royal Astronomical Society, November 21, 2014.

Searching for the perfect gift for the astronomer in your family? Whether they love black holes or prefer the stars they can see in the sky, you just might find something in our 12 Days of Deals special. Check back every day now through December 14th to see what we're offering!

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Carbon Stars Will Make You See Red

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Treasure hunting for carbon stars, the rubies of the night sky.

Carbon stars' color make them visual magnets

R Leporis in Lepus is one of the best known carbon stars. It shines an alluring red on winter evenings.
Joseph Brimacombe

Color can be tough to come by in the deep sky, especially if you own a small telescope. Planets serve up a medley of subtle hues, as do a few planetary and bright nebulae. Stars show tints of blue, yellow, and orange, but there's nothing quite like the color red. Show a yellow star to someone at a star party and you might hear a polite "that's nice."  But point the scope at a smoldering ruby like T Lyrae? Watch their faces light up. We react with instinctive pleasure at seeing radiant reds, and there's no better place to get our fill than carbon stars.

Most carbon stars are red giants, one of the reasons for their ruddy hue. Much of the star's red complexion comes from carbon in its atmosphere. Stars generate light and heat by nuclear reactions, converting hydrogen into helium in their cores. As a sun-like star ages, its core compresses and heats up until it can cook helium nuclei into carbon. Convective currents dredge carbon from the core and deliver it to the star's outer layers where it forms a fine soot that scatters away blue and green light. Only oranges and reds penetrate the dusty barrier to reach our eyes.

Stars burn fuels in stages

Artist’s impression of the structure of a sun-like star and red giant. Sun-like stars fuse hydrogen into helium for most of their lives before evolving into red giants. Giants fuse helium into carbon in their cores. Some of it can be delivered to the star's atmosphere where it forms carbon molecules, silicon carbide, and other carbon compounds. Image scale shown in the lower right corner.
ESO

Sunsets are red for a similar reason. At the horizon, sunlight takes a longer path through the denser, dustier air of the lower atmosphere compared to midday. Blues and greens are scattered away, leaving only the warmer colors.

All carbon stars are variable stars — the reason for their letter designations — and vary in brightness with periods that range from a couple months to more than a year. Perception of star color has much to do with a star's brightness. One star might be touted as redder than another, but if it's on the bright end of its cycle, shining at 6th or 7th magnitude, the color will look less saturated than it does when the star hovers near minimum. R Leporis, better known as "Hind's Crimson Star," is an intensely red ember when in the 9-10 magnitude range. Presently around magnitude 6.5, it looks washed out in comparison. Scope size also plays into carbon star color, making each person's experience different.

Color thesaurus for carbon star observers

Need some help describing the great variety of carbon star hues? Use the "red block" from Ingrid Sundberg's color thesaurus.
Ingrid Sundberg

Another peculiarity of observing carbon stars has to do with the eye's perception of red-hued objects in low light; this is called the Purkinje Effect. U Cygni, which glimmers at 10th magnitude this week, looked dim and nearly colorless when I first glanced at it. But the longer I stared, the redder and brighter it became. Now it's on my favorites list, along with S Cephei (cherry red!) and R Leporis.

Because color perception is subjective, astronomers don't use "red" or "blue" to describe star tints. Instead, they measures a star's magnitude or brightness through a "B" (blue) filter and a "V" (visual) filter (similar to what the human eye sees). The difference between the two magnitudes is called the B-V color index. Extremely hot stars pour out more blue light and have B-Vs that are generally less than zero; cooler red stars have positive indices. Blue-white Spica in Virgo has a B-V of –0.3, the Sun +0.65 (slightly yellow), and the carbon star R Leporis a whopping +5.7 (very red). Generally, the larger the number, the redder the star.

Context makes the sight that much sweeter

U Cygni (upper left) makes a beautiful contrast with an 8th magnitude white star against a delicate diffuse nebula. Lovely!
Greg Parker

Sometimes a carbon star will share the field with a star of a very different color for a striking contrast. Astrophotographer and red star aficionado Greg Parker of the U.K. is smitten by U Cygni's pairing with a nearby white star against the backdrop of a delicate red nebula.

Carbon stars are scattered across the sky, guaranteeing plenty of happy viewing no matter the season. I've selected a dozen for your perusal. Every one is up during evening hours this month except V Hydrae, which I've included because it's billed as one of the reddest stars in the sky. Don't miss it. 

Two coals smolder in the night

Two red stars lie near the pretty asterism Kemble's Cascade (crossing from lower left to upper right) in Camelopardalis. The lower star, marked here as SAO 12870, is better known as U Camelopardalis and has a B-V of +4.9. Click to visit Parker's website for more carbon star photos.
Greg Parker

Current magnitudes are shown in parentheses and estimated by myself or gleaned from the website of the American Association of Variable Star Observers (AAVSO). The AAVSO is a great place to print out finder charts for each of these gems. Click HERE and key in the star's name in the Pick a Star box. You can check recent brightness estimates by clicking Check Recent Observations. Every star listed is plotted on Wil Tirion's Sky Atlas 2000.0 as well. 

Watch out. You might get hooked on these rubies. If you do and crave more, the Astronomical League offers a carbon star observing program featuring a list of 100 carbon stars.

NameLocationB-VMagnitude rangePeriodNotesT Lyrae18h 32′, +36° 59′ +3.7      7.5-9.2 (8.9)IrregularVery deep red!UX Draconis19h 21′, +76° 33′ +3.5      5.9-7.1 (6.7)175 daysFiery orange-redU Cygni20h 19′, +47° 53′ +3.7      5.9-12.1 (9.8)463 daysDim, rich redS Cephei21h 35′, +78° 37′ +4.8      7.4-12.9 (9.8)487 daysDeep cherry red!TX Piscium23h 46′, +03° 29′ +2.5      4.8-5.2 (5.3)IrregularYellow-red, brightVX Andromedae00h 20′, +44° 42′ +1.6      7.5-9.7 (8.6)375 daysMedium redR Leporis4h 59′, –14° 58′ +5.7      5.5-11.7 (6.3)445 daysRed, brightBL Orionis6h 25′, +14° 43′ +2.4      5.9-6.6 (6.6)154 daysFiery orange-redUU Aurigae6h 36′, +38° 26′ +2.6      5.1-6.6 (5.5)234 daysFiery, bright!T Cancri8h 56′, +19° 51′ +5.3      7.6-10.5 (9.8)482 daysDeep orange-redV Hydrae10h 51′, –21° 15′ +4.5      6.0-12.3 (9.5)531 daysDeep copper-redY Canum Venaticorum12h 45′, +45° 26′ +2.9      4.9-5.9 (5.5)268 daysYellow-orange

Searching for celestial treasure? Let our Pocket Sky Atlas be your map!

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Hayabusa 2 is Asteroid Bound

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The Japanese asteroid-sample mission launched successfully.

On December 3rd, the Japan Aerospace Exploration Agency (JAXA) launched the spacecraft Hayabusa 2 on an epic journey — to an asteroid and back again. The intrepid asteroid explorer will rendezvous with near-Earth asteroid 1999 JU3 and obtain samples from its surface before returning to Earth.

Hayabusa 2 spacecraft

The Japanese Hayabusa 2 mission will travel to asteroid 1999 JU3 with a host of instruments to take samples and bring them back to Earth.
Akihiro Ikeshita / JAXA

The Hayabusa 2 payload was carried aloft by an H-IIA rocket, which took off from the Tanegashima Space Center in southern Japan at 1:22 p.m. JST (04:22 Universal Time), after suffering several days of delays due to bad weather.

Hayabusa 2 is the successor to the first Hayabusa mission, which was the first (and so far only) mission to bring samples of an asteroid back to Earth. The first Hayabusa limped home with particles from 25143 Itokawa in June 2010, after a harrowing number of problems. Hayabusa 2’s target asteroid, 1999 JU3, is about 900 meters (3,000 feet) in diameter and generally circles the Sun between the orbits of Earth and Mars, rotating with a period of about seven and a half hours.

1999 JU3 is a C-type, or carbonaceous, asteroid. C-type asteroids are the most common type of asteroid, dark gray, and often seen in the main asteroid belt’s outer regions. Planetary scientists find them especially interesting because they are thought to contain significant quantities of water and organic compounds. The origin of water on Earth is not fully understood, but many scientists think it was brought to Earth by carbonaceous fragments bombarding the primordial planet. Because asteroids formed early in the evolution of the solar system and have changed little since then, they may hold tantalizing clues from this era, 4.6 billion years ago. Analysis of the samples from 1999 JU3 could help scientists understand our watery Earth’s origins and reveal the types of organic compounds that formed in the early solar system.

Hayabusa 2 will be the third spacecraft to land on an asteroid (after NASA's NEAR-Shoemaker mission and the original Hayabusa mission) and (if it works) the second to return samples of an asteroid to Earth. The original Hayabusa mission studied an iron-rich S-type, or silicaceous asteroid, making Hayabusa 2 the first effort to bring back a sample from a C-type asteroid.

Hayabusa 2 instruments

The Hayabusa 2 mission includes the standard solar arrays and antennas but also carries a lander, rovers, and a deployable camera.
JAXA

Similar in design to its predecessor, the main body of Hayabusa 2 is 1 meter × 1.6 meter × 1.4 meter. It weighs in at a total of 600 kg (1,300 lbs), including fuel. When extended, its solar panels span 6 meters. Although Hayabusa 2 looks like its older sibling, the team has beefed up the technology aboard the spacecraft.

One new feature of Hayabusa 2 is a speeding bullet: the spacecraft’s Small Carry-on Impactor will use an explosive device to shoot a 2-kg copper projectile at the asteroid at a velocity of 2 km per second, creating an artificial crater. The spacecraft will deploy a camera to watch the impact (the craft itself will hide behind the asteroid), and the debris will be sampled to study pristine material from beneath the asteroid’s surface.

In addition to taking samples during brief touch-downs in three locations on the asteroid’s surface, Hayabusa 2 will study 1999 JU3 remotely, using its Near Infrared Spectrometer and Thermal Infrared Imager to examine the temperature variations and mineral composition of the asteroid. Hayabusa 2 will also bring along several traveling companions to make close-up observations. Three small MINERVA-II rovers (similar to the failed MINERVA from Hayabusa 1) and a lander, MASCOT, will take a variety of detailed measurements at the surface.

For now, Hayabusa 2 has a long journey ahead of it. After building up some speed swinging by Earth in 2015, Hayabusa 2 will meet up with 1999 JU3 in 2018 and stay there for 18 months, before returning home in 2020 — hopefully with a precious payload.

 

Shopping for the holidays? Check out the 12 Days of Deals in our online store www.shopatsky.com.

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Astrophotography Book by Thierry Legault

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Rocky Nook

802 E. Cota St., #3, Santa Barbara, CA 93103
805-687-8727; www.rockynook.com

NPS_Legault-book-300pxAuthor and world-renowned astrophotographer Thierry Legault teaches you the art and techniques of astro-imaging in his book Astrophotography ($39.95, or $31.95 in eBook format). Legault's compendium walks readers through the hows and whys of astronomical imaging, from basic camera-on-tripod skyscape photography to the more complex and demanding processes that use specialized telescopes and cameras for a variety of astronomical subjects, including the Sun, Moon, planets, artificial satellites, and deep-sky targets such as nebulae and galaxies. Legault shares his experiences to help you obtain the best results from a variety of equipment while guiding you through the common steps used to capture and process astronomical imagery. Paperback, 225 pages. ISBN 978-1-937538-43-9.

SkyandTelescope.com's New Product Showcase is a reader service featuring innovative equipment and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Sky & Telescope assumes no responsibility for the accuracy of vendors statements. For further information contact the manufacturer or distributor. Announcements should be sent to nps@SkyandTelescope.com. Not all announcements will be listed.

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Los Discorsi de Galileo - Primer día (II)

eltamiz -

Por si no sabes de qué va esto, nos encontramos leyendo juntos una traducción comentada de los Discorsi e dimostrazioni matematiche, intorno à due nuove scienze (Discursos y demostraciones matemáticas en torno a dos nuevas ciencias, la última obra de Galileo Galilei. Tras la introducción, dedicatoria y presentación empezamos con el primer día de los diálogos, en el que conocimos a Sagredo, Salviati y Simplicio, cuya conversación continuamos hoy.

Esta vez, como verás, entramos en Física con mayúsculas y Galileo muestra su absoluto genio experimental, así como su intuición. Para recordarte dónde estábamos en el diálogo –esto lo haré siempre– empezaremos con la última intervención del fragmento anterior. Ya intenté romper el texto en un sitio en el que hubiese un cambio de tercio, para que no hubiera demasiada discontinuidad, pero en cualquier caso no está mal recordar lo que estaban diciendo los tres amigos y seguir desde ahí. Reunámonos con ellos de nuevo:

Sagredo – Una vez atada la cuerda a un soporte por el extremo superior, podía colgarse de ella aferrándose al cilindro con ambas manos. La presión de la cuerda, atrapada entre el cilindro interior y el exterior, era tal que podía, según le conviniese, aferrar el aparato con más fuerza y así evitar deslizarse hacia abajo, o aflojar las manos ligeramente y así descender tan suavemente como lo deseara.

Salviati – ¡Un aparato realmente ingenioso! Creo, sin embargo, que para obtener una explicación completa deberíamos tener en cuenta otras consideraciones; pero no debo desviarme en este asunto particular, ya que esperáis escuchar lo que pienso acerca de la resistencia a la rotura de otros materiales que, a diferencia de las cuerdas y la mayor parte de las maderas, no tienen una estructura filamentosa. La cohesión de estos cuerpos es, en mi estimación, producida por otras causas que pueden agruparse en dos clases.

Una es la tan mencionada repugnancia que exhibe la Naturaleza respecto al vacío; pero este horror al vació no es suficiente, y es necesario introducir otra causa en forma de una sustancia pegajosa o viscosa que una firmemente las diferentes partes que componen el cuerpo.

Recuerda que la primera ciencia de las dos del libro es la resistencia de materiales. Es esencial para Galileo, por tanto, intentar comprender por qué se mantiene la cohesión de sustancias como el granito. Su explicación del entrelazamiento de pequeñas fibras en la madera, cuerdas y otras sustancias de origen vegetal no le vale ahora; y, por supuesto, no existe aún una química que explique ningún tipo de atracciones eléctricas entre iones o moléculas polares.

De hecho, Galileo no dispone del conocimiento necesario para desentrañar el misterio, pero sí establece las bases fundamentales para que otros, en siglos posteriores, lo hagan. Como verás, su genio consiste en razonar a partir de los hechos observados, determinando características peculiares en el comportamiento de las cosas que permiten descartar posibles explicaciones, clasificar las causas que podrían explicar ese comportamiento… en definitiva, hacer ciencia. Así, el italiano empieza por distinguir dos razones por las que algo podría estar aparentemente pegado a otra cosa, de modo que cada una de las dos razones explique determinadas situaciones reales que no puede explicar la otra.

En primer lugar hablaré sobre el vacío, demostrando con un experimento definitivo la calidad e intensidad de su fuerza. Si se toman dos platos lisos y muy pulidos de mármol, metal o vidrio y se los pone cara a cara, uno se deslizará sobre el otro con enorme facilidad, lo cual muestra de manera concluyente que no hay nada de naturaleza viscosa entre ellos. Pero cuando se intenta separarlos y mantenerlos a una distancia constante entre sí, se observa que los platos muestran una repugnancia a la separación de modo que el superior se lleva al inferior consigo y lo mantiene elevado indefinidamente, incluso aunque el segundo sea grande y pesado.

¿Ves a lo que me refería con lo de analizar situaciones para descartar explicaciones posibles? Las uniones “pegajosas” no pueden ser las únicas existentes en la Naturaleza, ya que al poner dos superficies muy lisas una sobre otra, y asegurarse de que todo el aire ha salido entre ellos –la manera más fácil es mojar las superficies–, cuesta separar las superficies. Por lo tanto, algo trata de mantenerlas unidas.

Pero ese algo no puede ser ninguna fuerza de atracción entre las superficies, como la que originaría la resina o el pegamento, ya que no cuesta absolutamente nada deslizar las superficies: debe haber necesariamente algún otro tipo de fuerza que no se opone al movimiento relativo de las superficies, sino a su separación. Y esa fuerza es el horror vacui, claro. Pero aquí Galileo sigue cuestionando el conocimiento establecido, como veremos luego.

Este experimento demuestra la aversión de la Naturaleza al espacio vacío, incluso durante el breve momento requerido para que el aire externo penetre en la región entre los dos platos. También se observa que si los dos platos no están suficientemente pulidos su contacto es imperfecto, de modo que cuando se trata de separarlos poco a poco la única resistencia ofrecida es la del peso; sin embargo, si el tirón es repentino, entonces el plato inferior se eleva, pero rápidamente cae de nuevo, habiendo seguido al plato superior únicamente durante el brevísimo intervalo de tiempo requerido para la expansión de la pequeña cantidad de aire entre los dos platos, debido a que no encajan, y para la entrada de aire del exterior. Esta resistencia que se observa entre los platos indudablemente existe entre las diferentes partes de un sólido y es, al menos en parte, una causa fundamental de su cohesión.

Dicho de otro modo, sería posible tener un material entre cuyas partes –macroscópicas o microscópicas, aunque Galileo no usara esos términos– se mantengan cohesionadas sin que haya atracciones entre ellas: el horror vacui podría ser suficiente para mantenerlas juntas. El italiano no piensa que ésa sea la causa última de la cohesión de las rocas, por ejemplo, pero sí que el efecto puede contribuir a la cohesión de los materiales. Al fin y al cabo muchas sustancias no tienen aire dentro, y sus partes por tanto se comportarán como esos dos platos del ejemplo.

Pero ahora viene el cuestionamiento, a través de varios personajes, del concepto clásico del horror vacui. Por si no conoces los detalles de la idea clásica, te resumo las dos ideas fundamentales:

  • La Naturaleza aborrece el vacío de tal modo que el vacío nunca puede existir. Donde existiría se moverá algo para rellenar el posible hueco.

  • Este horror al vacío es absoluto: nada puede superarlo. En términos más modernos, no hay fuerza que pueda vencerlo.

O eso decía Aristóteles, claro…

Sagredo– Permite que te interrumpa un momento, por favor; pues quiero hablar sobre algo que se me acaba de ocurrir, el hecho de que cuando veo al plato inferior seguir al exterior y elevarse rápidamente estoy seguro de que, contrariamente a la opinión de muchos filósofos, incluso del propio Aristóteles, el movimiento en el vacío no es instantáneo. Si esto fuera así, los dos platos mencionados arriba se separarían sin la menor resistencia, ya que el mismo instante de tiempo serviría para su separación y para que el medio circundante entrase y llenase el espacio entre ellos.

El hecho de que el plato inferior sigue al exterior nos permite inferir, no sólo que el movimiento en el vacío no es instantáneo, sino también que entre los dos platos existe realmente un vacío, al menos durante un corto espacio de tiempo, el suficiente para que el medio circundante entre y rellene ese vacío; pues si no hubiera un vacío no habría ninguna necesidad de que se moviera el aire. Uno debe admitir, por lo tanto, que a veces se produce un vacío mediante un movimiento brusco o contrario a las leyes de la Naturaleza (aunque en mi opinión nada sucede contrario a la Naturaleza excepto lo imposible, y eso nunca sucede).

El razonamiento es cuestionable: Galileo no demuestra fehacientemente que sí pueda existir el vacío. Pero, por otro lado, ¡observa lo revolucionario de la última idea, tan brevemente expuesta! Anteriormente se hablaba constantemente de movimientos o fuerzas naturales y forzados –contrarios a lo natural–. Pero Galileo, con una clarividencia total, expone una idea mucho más moderna: todo lo que sucede en el mundo es natural. Dicho de otro modo, todo lo que sucede lo hace de acuerdo con leyes naturales comprensibles y universales. Si algo fuera contrario a la Naturaleza, no sucedería. No hay distinción entre movimientos producidos por una persona o el viento.

Pero aquí surge otra dificultad. Aunque el experimento me convence de la corrección de esta conclusión, mi mente no está completamente satisfecha con la causa a la que se atribuye este efecto. La separación de los platos precede a la formación del vacío que se produce a causa de esta separación; y puesto que me parece que, en el orden de la Naturaleza, la causa debe preceder al efecto, aunque parezcan suceder en el mismo instante, y puesto que cualquier efecto positivo debe tener una causa positiva, no veo cómo la adhesión entre los dos platos y su resistencia a la separación –hechos reales– pueden referirse a un vacío como causa, cuando este vacío aún no se ha producido. De acuerdo con la máxima infalible del Filósofo, lo que no existe no puede producir ningún efecto.

Por si esta idea es liosa, permite que la exprese con mis palabras. La cadena causa-efecto es supuestamente la siguiente: los platos se mantienen unidos porque no puede existir vacío entre ellos. Al separarse aparecería vacío entre ellos, lo cual no puede existir, de modo que los platos se resisten a separarse. Pero claro, si nunca llega a existir vacío entre los platos, ¿cómo puede ese vacío ser responsable de que no se separen?

En mi opinión, sin embargo, este cuestionamiento por parte de Sagredo está tomado con alfileres. Y no creo que fuera en este caso la voz de Galileo, ya que Simplicio pone de manifiesto lo endeble del razonamiento. Estamos dando vueltas a los conceptos de un modo un poco tonto, pero en breve Galileo introducirá la ciencia experimental en el asunto de un modo genial.

Observa también cómo Simplicio, lejos de ser el tonto que fue en otros libros del italiano, tiene aquí intervenciones en las que es quien se cuestiona las cosas del modo más agudo y corrige a los otros:

Simplicio– Viendo que aceptas este axioma de Aristóteles, dudo que rechaces otra máxima suya, excelente y muy fiable: la de que la Naturaleza sólo emprende lo que se produce sin resistencia; y en esta afirmación, me parece a mí, encontrarás la solución a tu problema. Ya que la Naturaleza aborrece el vacío, evita aquello que produciría el vacío como consecuencia necesaria. Por esa razón la Naturaleza evita la separación de los dos platos.

Sagredo– Admitiendo que lo que dice Simplicio es una solución adecuada a mi problema, me parece, si se me permite retomar mi argumento anterior, que esta misma resistencia al vacío debería ser suficiente para sostener las partes constituyentes de la piedra o el metal, o las partes de cualquier otro sólido unido con mayor cohesión y que es más resistente a la separación. Si puede haber una sola causa para un efecto o si, al asignar otras, pueden reducirse todas a una, ¿por qué no es este vacío, que realmente existe, una causa suficiente para todos los tipos de resistencia?

La idea de Sagredo es la del principio de parsimonia de Ockham: “Numquam ponenda est pluralitas sine necessitate (Nunca debe postularse la pluradidad sin necesidad).” En palabras del más grande de los sucesores de Galileo, Sir Isaac Newton, “We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances (No debemos admitir más causas para las cosas naturales que las que son verdaderas y suficientes para explicar su comportamiento)”. En este caso sí es necesario postular una segunda causa para la cohesión de los cuerpos, pero el divino italiano quiere recordarnos que no lo hace por ignorancia del principio de parsimonia, sino porque es inevitable hacerlo de acuerdo con él.

Salviati– No deseo entrar ahora mismo en esta discusión sobre si el vacío es suficiente por sí mismo para mantener cohesionadas las diferentes partes de un sólido; pero te aseguro que el vacío que actúa como causa suficiente en el caso de los dos platos no es suficiente por sí mismo para mantener unidas las partes de un cilindro sólido de mármol o metal los cuales, cuando se tira de ellos violentamente, se separan y dividen. Ahora bien, si encuentro un método de distinguir esta bien conocida resistencia que depende del vacío de cualquier otra causa que aumente la cohesión, y si te muestro que la resistencia anteriormente mencionada no es suficiente por sí misma para tal efecto, ¿no admitirás que necesitamos introducir alguna otra causa? Ayúdalo, Simplicio, ya que no sabe cómo responder.

Simplicio– Indudablemente la vacilación de Sagredo se debe a alguna otra razón, porque no puede existir ninguna duda sobre una conclusión a la vez tan clara y lógica.

Sagredo– Lo has adivinado, Simplicio. Me estaba preguntando si no sería necesario, si no fuera suficiente un millón en oro español al año para pagar al ejército, hacer acopio de alguna otra cosa que no fueran monedas de pequeño valor para la paga de los soldados. Pero continúa, Salviati; supongamos que admito tu conclusión, y muéstranos tu método de separar la acción del vacío de otras causa, y midiéndola, demuéstranos que no es suficiente para producir el efecto en cuestión.

Esta misteriosa afirmación de Sagredo, que no viene a cuento (y Salviati hace mención a ello encomendando a Sagredo a su Ángel de la Guarda), tiene que ver con algo que se dirá después. Por ahora simplemente olvídate de ella, porque no afecta al razonamiento sobre el vacío y los materiales. Para compensar de esta salida de tema, en la siguiente intervención de Salviati hay una afirmación revolucionaria, a ver si la detectas:

Salviati– Que tu Ángel de la Guarda te proteja. Os explicaré cómo separar la fuerza del vacío de las otras, y posteriormente cómo medirla. Para este propósito, pensemos en una sustancia continua cuyas partes carecen de cualquier resistencia a la separación excepto la derivada del vacío, como sucede con el agua, un hecho completamente demostrado por nuestro Académico en uno de sus tratados.

Salviati lo deja caer así, como si nada: medir la fuerza del vacío. Pero si es posible medir algo, es porque no es infinito, como decía Aristóteles. Aunque no lo esté diciendo explícitamente, Galileo deja bien claro que es de la opinión de que el horror vacui no es absoluto. Y como es más chulo que nadie, se dispone a demostrarlo con otro experimento mental. La descripción es liosa, pero la examinaremos paso a paso:

Cuando un cilindro de agua es sometido a tensión y ofrece resistencia a la separación de sus partes, esto sólo puede ser atribuido a la resistencia del vacío. Para realizar un experimento de este tipo he inventado un aparato que puedo explicar mejor con un esbozo que con meras palabras. Sea CABD el corte de un cilindro de metal o, preferiblemente, de vidrio, hueco por dentro y fabricado cuidadosamente.

Figura 4.

Se introduce en él un cilindro de madera que encaje perfectamente, representado por la sección EGHF, y capaz de movimiento vertical hacia arriba y abajo. A través del centro de este cilindro se taladra un agujero para introducir por él un cable de hierro con un gancho en el extremo K, mientras que el extremo superior del cable I tiene una cabeza cónica. El cilindro de madera tiene una depresión en la parte superior para recibir, encajando perfectamente, la cabeza cónica I del cable de metal cuando se tira hacia abajo del extremo K.

Como ves, lo que describe Galileo es un pistón: un cilindro hueco externo, con un émbolo interior de madera. Lo peculiar son los detalles: el gancho de metal del que cuelga un tubo, que tiene una parte superior engrosada que encaja en una depresión del émbolo de madera, y el hueco V en el cilindro exterior. Pero estos detalles también tienen una razón de ser, y es posible que ya te estés imaginando lo que pretende el italiano con este pistón del que cuelga un peso.

A continuación se inserta el cilindro de madera EGHF en el cilindro hueco CABD, de modo que no se toque el extremo superior del segundo, sino que se deje un espacio libre de dos o tres dedos; este espacio debe llenarse de agua dando la vuelta al recipiente de modo que la boca CD mire hacia arriba, empujando hacia abajo el émbolo EH y, al mismo tiempo, evitando que la cabeza cónica del cable metálico I toque la parte hueca del cilindro de madera. Así se permite que el aire escape por el agujero central junto al cable metálico –que no encaja perfectamente en el agujero– tan pronto como se presiona el émbolo hacia abajo.

Ves ya la razón de que exista la cabeza engrosada I. El hueco central del cilindro de madera permite que entren y salgan el aire o el agua, pero si se tira del cubo hacia abajo, la cabeza I encaja en su hueco y hace de tapón hermético. En este paso, Galileo ha descrito cómo llenar de agua el hueco interior del pistón: al invertir todo el aparato y verter agua por el agujero central del émbolo, el agua desplazará todo el aire del interior, hasta rebosar por el hueco central. En ese momento no quedará nada de aire dentro del pistón.

Una vez que ha escapado el aire del interior y tirando del cable para que la cabeza cónica encaje en el hueco de la madera, se invierte de nuevo el aparato de modo que la boca CD mire hacia abajo, y se cuelga del gancho K un recipiente que puede llenarse de arena o cualquier material pesado en la cantidad necesaria para conseguir separar finalmente la superficie superior del émbolo, EF, de la superficie inferior del agua a la que estaba unido tan sólo por la resistencia del vacío. A continuación se pesan el émbolo y el cable junto con el recipiente que cuelga de ellos y su contenido: tendremos entonces la fuerza del vacío.

Galileo usará esto para hablar sobre los materiales, pero debemos pararnos un momento porque esto es, una vez más, revolucionario, aunque el italiano no le dé la importancia que merece.

Al cerrar el hueco herméticamente con la cabeza I, se garantiza que todo el hueco del pistón está lleno de agua. Si se da la vuelta al aparato y se tira hacia abajo del gancho metálico, se notará que el émbolo no baja, como si estuviera pegado arriba. La explicación clásica es clara: si tiramos hacia abajo del émbolo, dado que nada puede entrar dentro y aumentamos el volumen del hueco interior, aparecería el vacío. Pero eso es imposible, ya que la Naturaleza aborrece el vacío, y ésa es la causa de la resistencia infinita del émbolo a bajar.

Pero, como ves, Galileo no piensa lo mismo –y puedo asegurarte que lo piensa tras haber construido este émbolo y haber hecho el experimento–: si se tira con suficiente fuerza del gancho, el émbolo se separará del agua. Y eso significa algo crucial: es posible vencer la resistencia causada por el vacío, y es posible generar un vacío. El vacío puede existir dentro del pistón.

Y hay más: el italiano no se conforma con tirar más y más del gancho hasta separar el pistón, demostrando así que es posible hacer una fuerza tan grande que se logre el vacío. Lo hace añadiendo más y más peso al cubo, porque de ese modo puede medir la fuerza del vacío experimental y objetivamente. Tremendo.

Desde luego, podría haber pegas a este experimento (¿no se expandirá el agua? ¿no entrará aire por algún sitio?), pero si crees que Galileo no está preparando una respuesta a esas pegas es que aún no lo conoces. Por ahora, enlacemos esto con la cohesión de los materiales:

Si se une a continuación a un cilindro de mármol o vidrio un peso que, al sumarlo al del propio trozo de mármol o vidrio, sea exactamente igual a la suma de los pesos antes mencionados, y si el cilindro se rompe, estaremos justificados en afirmar que el vacío por sí mismo es capaz de mantener unidas las partes del mármol o el cristal; pero si este peso no es suficiente y el cilindro sólo se rompe tras añadir, por ejemplo, cuatro veces este peso, deberemos entonces llegar a la conclusión de que el vacío sólo es responsable de la quinta parte de la resistencia total.

Dicho de otro modo, ahora que tenemos una medida de la fuerza del vacío, sabemos lo máximo que podría cohesionar esa fuerza: un peso igual al que separa el cilindro del pistón, siempre que el cuerpo estudiado tenga el mismo diámetro. Si hace falta más fuerza para cohesionar un cuerpo, el vacío no puede ser el único responsable; de otro modo, podría ser el único responsable: no tiene por qué serlo, pero sí podría bastar para explicarlo, lo cual enlaza con el principio de parsimonia de antes.

Simplicio –Nadie puede dudar de la ingeniosidad de este aparato. Sin embargo, presenta muchos obstáculos que me hacen dudar de su fiabilidad. ¿Quién puede asegurarnos, por ejemplo, que el aire no penetrará entre el vidrio y el émbolo incluso si se introduce entre ellos estopa o algún otro material similar? También me pregunto si utilizar cera o aceite de trementina bastará para conseguir que el cono I encaje perfectamente en su asiento. Además, ¿no se expandirán y dilatarán las partes del agua? ¿Qué impide que entre aire o exhalaciones de alguna sustancia más sutil a través de los poros de la madera o incluso del propio vidrio?

Observa la meticulosidad de Galileo como experimentador. Se plantea lo evidente (que entre aire entre las paredes del émbolo y el pistón), pero también lo sutil: que existan poros microscópicos por los que pueda entrar aire, y que la densidad del agua no tenga por qué ser constante y el líquido pueda expandirse hasta rellenar todo el hueco, evitando así la aparición del vacío. Y a la vez sugiere la solución para algunos de los problemas evidentes –una vez más, con seguridad, el italiano lo había hecho ya físicamente antes de hablar de ello–.

Salviati –Simplicio nos ha mostrado con gran habilidad los obstáculos; e incluso ha sugerido parcialmente cómo evitar que el aire penetre en la madera o entre la madera y el vidrio. Pero permitidme que señale que, según aumenta nuestra experiencia, sabremos si estos presuntos obstáculos existen o no. Porque si el agua, como le sucede al aire, es de naturaleza dilatable, aunque sólo sea mediante tratamientos extremos, veremos que el émbolo desciende.

No entiendo la descripción de esta salvaguarda, y espero que alguno de vosotros pueda ayudarme. Lo que entiendo es: si el agua puede dilatarse, aunque cueste mucho más conseguirlo que con un gas, al tirar del émbolo el agua disminuiría su densidad y rellenaría el hueco. El émbolo entonces descendería. Pero lo que no entiendo es: ¿cómo se distingue esto del descenso del émbolo dejando vacío detrás? Galileo parece convencido de que la dilatación del agua está descartada, pero no entiendo por qué.

Y si hacemos una pequeña hendidura en la parte superior del recipiente de vidrio, como se indica en V, entonces el aire o cualquier otra sustancia tenue y gaseosa que pudiera penetrar por los poros del vidrio o la madera, pasaría a través del agua y se acumularía en este receptáculo V. Pero si no sucede ninguna de estas cosas, podremos estar seguros de que nuestro experimento ha sido realizado con la precaución adecuada; y descubriremos entonces que el agua no se dilata y el vidrio no permite que ningún material, no importa lo tenue que sea, lo atraviese.

Ésta es la razón de que la pared interior del pistón no sea plana, sino que haya un hueco V: cuando el cubo cuelga del gancho y el hueco V está arriba, ese receptáculo almacenaría cualquier cosa que no fuera agua pero sí menos densa que el agua. De este modo, si entrase aire a través de poros en el pistón o del hueco entre émbolo y pared, el aire subiría por su menor densidad, rellenaría el hueco V, dejaría de haber vacío alguno y el émbolo caería sin resistencia –o con una resistencia muy pequeña– hasta abrir el pistón completamente.

Galileo se defiende así, antes siquiera de publicar el libro, de los comentarios de quienes rápidamente buscarían pegas a su experimento – porque es seguro que sucederá. Desde luego es posible encontrar otras, pero es curioso como el italiano es muy cuidadoso con el rigor de sus experimentos, y ataca él mismo sus posibles pegas para descartarlas, ya que de otro modo la “revisión por pares” lo descuartizará vivo. Una vez más, ciencia moderna en pañales, pero ciencia moderna.

En el siguiente episodio seguiremos hablando del vacío – ya ves que no es casualidad que Evangelista Torricelli fuese discípulo de Galileo. ¡Hasta entonces!

Lick Observatory Gets a Reprieve

Sky&Telescope -

Last year the University of California ordered its astronomers to make historic Lick Observatory self-supporting by 2018. Now there's been a change of heart, and the university will continue to pay for its operation.

Lick Observatory (aerial view)

Lick Observatory sits atop 4,216-foot-high Mount Hamilton and overlooks San Jose and Silicon Valley in California.
Debra and Peter Ceravolo

A year ago, the situation looked bleak for historic Lick Observatory, the venerable 125-year-old mountaintop facility that overlooks California's Silicon Valley.

Faced with huge commitments to support its investment in Hawaii's Keck Telescopes and to help fund the billion-dollar Thirty Meter Telescope, officials at the University of California (which owns and operates Lick) decided there just wasn't enough money to go around. So they decreed that Lick should be divested from the university and find its own funding, with a "glide path" toward self-sufficiency to begin within two years and be completed by 2018.

Needless to say, the September 2013 announcement rocked the astronomical community in a way that few of the area's earthquakes ever could. Although "closure" was never actually stipulated, it loomed as the most likely outcome for a venerable institution that held a premier role in U.S. astronomy a century ago.

Prominent members of the astronomical community cried out in protest. Cosmologist Alex Filippenko (UC Berkeley) led a "Save Lick Observatory" campaign. California congressmen petitioned the university's president to reconsider the decision. The area's amateur astronomers mobilized for a fight.

Apparently, all that high-profile resistance — coupled with some belt-tightening — has spared Lick from being cast adrift. In a letter to Claire Max, interim director of University of California observatories (UCO), provost Aimée Dorr rescinded the divestment plan. "It is no longer [our] intention to require that Lick Observatory be self supporting, begin a glide path to self-supporting status no later than FY 2016-17, or be managed by an entity other than UCO," the letter states.

While the observatory's short-term prospects are now relatively secure, proponents are taking steps to ensure its long-term survival. A Lick Observatory Council, involving Filippenko, other scientists, and private citizens, has started private fundraising efforts and to expand the observatory's outreach and education programs.

Once the World's Largest TelescopeLick's Great Refractor

The huge refractor at Lick Observatory in California boasts an objective lens 36 inches across. James Lick, who funded the observatory's construction in the late 1800s, is buried in the telescope's massive base.
Debra and Peter Ceravolo

Built in the late 1800s thanks to a $700,000 bequest from wealthy tycoon James Lick, the observatory sits atop California's Mount Hamilton, just 30 miles southeast of San Francisco. To its immediate west is San Jose, which was a quiet town of about 13,000 back then but is now a city of a million souls at the southern end of burgeoning Silicon Valley.

This was the first mountaintop observatory to be continuously occupied, and it's been under the University of California's management since 1888. Light pollution rendered the facility useless for most scientific research long ago, though in recent years Lick's 3-meter Shane Telescope has been at the forefront of the search for exoplanets.

Most visitors don't want to see that instrument, however — they come to see the magnificent 36-inch refractor, whose objective was fabricated in Massachusetts by Alvan Clark & Sons. It's 57 feet long, 4 feet across, and weighs more than 25,000 pounds. For a few years, it was the world's largest telescope — until Clarks finished work on the 40-inch objective for Yerkes Observatory in Wisconsin.

Historical note: James Lick originally intended to build a giant pyramid in downtown San Francisco but was persuaded to use his wealth instead to build a telescope "superior to and more powerful than any telescope yet made." He chose Mount Hamilton so he could see the observatory from his home. He died more than a decade before its completion, however, and his body is interred under the great refractor that bears his name.

Want a telescope of your own but don't want to spend $700,000 on it? Check out the dozens of offerings showcased in Sky & Telescope's Test Reports.

The post Lick Observatory Gets a Reprieve appeared first on Sky & Telescope.

Tour December’s Sky: Orion Rising

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Our monthly podcast offers the key highlights for stargazing in December: where to find bright stars and planets — and how to spot the Geminid meteor shower.

Another year is drawing to a close. On December 21st at 6:03 p.m. Eastern Standard Time we reach a celestial turnaround point: the solstice, a Latin word meaning “Sun stands still.” At that moment the Sun appears farthest south in the sky, standing directly over Earth’s Tropic of Capricorn, and reverses direction to return gradually to northern declinations. This date marks the astronomical beginning of summer in the Southern Hemisphere and of winter up here in the north.

For stargazing, it’s a time of transition as well. The planets Mercury, Venus, and Saturn are hidden in twilight as December begins, though by month’s end Venus barely peeks over the western horizon at dusk and Saturn emerges in the east before dawn. Mars is still with us, low in the southwest after sunset, and Jupiter rises in late evening.

Orion rising

The distinctive stars of Orion climb up over the eastern horizon early on December evenings.
Sky & Telescope diagram

Leading the way for Jupiter is majestic Orion, which veritably leaps up over the eastern horizon around 7 pm at the beginning of December but soon after sunset by month’s end. You’ll be able to spot the Hunter’s distinctive three-star belt, oriented as a vertical row as the constellation climbs into the sky. The belt is flanked by ruddy Betelgeuse to its left and icy-white Rigel to its right.

December is also the month that features the return of one of the year’s very best meteor showers: the Geminids. From a clear, dark location, you might see one meteor per minute or more when it peaks on the nights of December 13th and 14th.

Get more tips on viewing the Geminids — and a personal tour of the stars and constellations overhead on December evenings — by downloading the 5½-minute-long stargazing podcast below.

http://www.skyandtelescope.com/wp-content/uploads/SkyTour-December-2014.mp3
Download the podcast here.

There's no better guide to what's going on in nighttime sky than the December issue of Sky & Telescope magazine.

The post Tour December’s Sky: Orion Rising appeared first on Sky & Telescope.

Spectra-L200 Slit Spectrograph

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JTW Astronomy

Aalsmeerderweg 103M, 1432CJ, Aalsmeer, Noord-Holland, The Netherlands
www.jtwastronomy.com

NPS_L200Spectrograph_300pxJTW Astronomy announces the Spectra-L200 ($1,830), a slit spectrograph for amateur telescopes. Based on the Littrow spectrograph design, the Spectra-L200 allows users with modest telescopes to produce the high-resolution spectra needed to explore the structure and chemical makeup of stars and bright nebulae, or to see the redshift of distant quasars. A custom, multireflective entrance slit plate provides a unique arrangement of nine different slit gaps, ranging from 20 to 100 microns, and three pinholes that you can quickly select using a built-in thumbwheel. The unit attaches to your telescope using a female T-thread and weighs 2.7 pounds (1.2 kilograms). It performs best with telescopes having focal ratios of f/7 or greater. The heart of the instrument is a reflective grating positioned behind an oversized achromatic doublet. Its highly reflective chromium surface and transfer mirror enable you to directly track the target star through the guide port with your autoguiding camera. Additional gratings and accessories are available through the manufacturer's website.

SkyandTelescope.com's New Product Showcase is a reader service featuring innovative equipment and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Sky & Telescope assumes no responsibility for the accuracy of vendors statements. For further information contact the manufacturer or distributor. Announcements should be sent to nps@SkyandTelescope.com. Not all announcements will be listed.

The post Spectra-L200 Slit Spectrograph appeared first on Sky & Telescope.

This Week’s Sky at a Glance, Nov. 28 – Dec. 6

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Some daily sky sights among the ever-changing Moon, planets, and stars.With a leg up over the trees, Orion announces winter's approach

The grand figure of Orion decorates the southeastern sky a half hour earlier every week. (Bob King photo.)

Friday, November 28

First-quarter Moon (exactly first-quarter at 5:06 a.m. Saturday morning EST). Look this evening for Fomalhaut far to the Moon's lower left, and for Enif, the nose of Pegasus, almost as far to the Moon's upper right.

Saturday, November 29

The Moon stands high in the south soon after nightfall, with the western side of the Great Square of Pegasus pointing down at it from above.

Sunday, November 30

It's the last day of November — so by 7 or 8 p.m. (depending on where you live) Orion is already clearing the eastern horizon, and by 9 or 10 p.m. it's striding well on its way upward in the southeast, as shown here.
By 10 or 11 p.m., the dim Little Dipper hangs straight down from Polaris.

Monday, December 1

Just as twilight fades away and night begins, look southwest for Mars. Above it by 10° — about a fist-width at arm's length — are Alpha and Beta Capricorni, both 3rd magnitude. Alpha, pale yellow-orange, is the one on the upper right. With sharp or well-corrected vision you can just make out that it's double. Binoculars split it very plainly and show other stars making an interesting asterism with it.

Binoculars also resolve more difficult Beta, whose secondary star is both fainter and closer in (to the bright component's lower right).

Tuesday, December 2

The Moon shines in the southeast soon after dark. Look below it, by a bit less than a fist at arm's length, for Alpha Piscium, the 3.8-magnitude star in Pisces that traditionally marks where the cords from the two fishes' tails are tied in a knot. Its name, Alrescha, is from the Arabic for "the rope," based on the ancient Greek description of the constellation.

Also this evening: Telescope users can watch the Moon's thin, invisible dark limb occult (black out) the 4.3-magnitude star Omicron Piscium as seen from the eastern half of North America except the southeast. Map and timetables.

Wednesday, December 3

Look about a fist and a half above the Moon this evening for the brightest stars of little Aries. Farther left of the Moon are the Pleiades, and much farther left sparkles bright Capella.

Watch night to night as the bright Moon passes Aldebaran, then Orion.

Watch night to night as the bright Moon passes Aldebaran, then Orion. (The Moon here is always drawn three times its actual apparent size.)

Thursday, December 4

Today brings the year's earliest end of evening twilight (if you're near 40° north latitude): at 6:11 p.m. if you live right on your time zone's standard meridian at that latitude. But the difference from day to day right now is very slight.

Friday, December 5

The Moon is essentially full both this evening and Saturday evening (exactly full at 7:27 a.m. Saturday morning EST.) On Friday evening in the Americas, the Moon shines less than about 2° from Aldebaran.

Saturday, December 6

By mid-evening the Moon shines high in the east. It's in a starry part of the sky. Aldebaran is now to its upper right. To the Moon's lower right is Aldebaran-colored Betelgeuse. Much farther lower left of the Moon are Castor and Pollux. High to the Moon's upper left shines brighter Capella.

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Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby; for an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

Pocket Sky Atlas

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's still less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability, which means fairly heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet RoundupJupiter and Regulus in the dawn, early December 2014.

Jupiter and Regulus in early dawn this week.

Mercury is hidden in the glare of the Sun.

Venus is buried very deep the sunset.

Mars (magnitude 1.0) still glows in the southwest during and after twilight. It sets around 8 p.m. local time.

Jupiter (magnitude –2.2, in western Leo) rises in the east-northeast around 10 or 11 p.m. About 40 minutes later, fainter Regulus (magnitude +1.4) rises below it. By dawn they shine high in the south, with Regulus now to Jupiter's left, as shown here.

Saturn is deep in the glow of sunrise.

Uranus (magnitude 5.8, in Pisces) and Neptune (magnitude 7.9, in Aquarius) are high in the southeast and south, respectively, right after dark. They move westward as the evening progresses. You'll need binoculars or a small telescope and our finder charts for Uranus and Neptune.

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All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 hours.

"Science is built up of facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house."

— Henri Poincaré (1854–1912)

The post This Week’s Sky at a Glance, Nov. 28 – Dec. 6 appeared first on Sky & Telescope.

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