Skylights featured on Astronomy Picture of the Day

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Skylights featured nine times on Earth Science Picture of the Day: 1 2 3 4 5 6 7 8 9


Photo of the Week.The top of the 22-degree halo around the Sun caused by refraction of sunlight through hexagonal ice crystals.

Astronomy news for the week starting Friday, January 9, 2015.

Phone: (217) 333-8789
Prepared by Jim Kaler.

Clear skies and thanks to Skylights' blogger visitor reader.

Go to STARS for previous stars of the week. Access Skylights' Archive and photo gallery. From the Sun to the Stars: the OLLI Lectures provides a linked, illustrated introduction to astronomy.
The Constellations has a linked list with locations and brightest stars. Constellation Maps show the locations of the constellations. The 170 Brightest Stars lists them through magnitude 3.00. For more on stars and constellations, visit Stellar Stories.
Tour the Milky Way. Watch a total eclipse of the Moon and an annular eclipse of the Sun. Moon Light presents scenic photos of the Moon. Go to MoonScapes for labelled telescopic images of the Moon and other lunar information.
See the Moon move and pass just below Nu Virginis. Watch planets move against the background stars. See a classic proof of the curvature of the Earth with a "hull down" series. Visit Measuring the Sky to learn about the celestial sphere.
Admire sunsets, rainbows, and other sky phenomena in Sunlight. Read the illustrated Day Into Night on the phenomena of the sky See the The Aurora and the Midnight Sun. See and understand the ocean tides.
Enjoy Our Complex Universe: A Human Understanding through Art, with 12 illustrations. Advances in Astronomy, 1989-2011. Take a ride aboard Asteroid 17851 Kaler (1998 JK). Look for Books about the sky and stars.


ASPSupport science literacy by joining the Astronomical Society of the Pacific, an international organization that is among the world's premier providers of astro education. Get Mercury and a variety of other benefits.

Presenting three audio courses with 70 to 100-page study guides, narrated and written by Jim Kaler.
Heavens Above: Stars, Constellations, and the Sky from Recorded Books. Astronomy: Earth, Sky, and Planets, is available from Recorded Books. Astronomy: Stars, Galaxies, and the Universe, is also now available from Recorded Books.
Astronomy: Earth, Sky, and Planets is published as Vault of the Heavens: Exploring the Solar System's Place in the Universe by Barnes and Noble.

Enjoy Our Complex Universe:A Human Understanding through Art, with 12 illustrations.

Read "Heaven's Touch: From Killer Stars to Seeds of Life, How We Are Connected to the Universe," Princeton University Press.

SSTo learn about stellar spectra, read STARS AND THEIR SPECTRA: An Introduction to the Spectral Sequence, Second Ed., with two new chapters and 140 new illustrations, Cambridge University Press (UK or North America), 2011.

Read From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.

SSNEWEST! FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. Read the interview with Jim Kaler.

NEW! Read Under the Handle in Stellar Stories.

This week centers on the third quarter Moon, which takes place on Tuesday, January 13, shortly before daybreak in North America, allowing a near-perfect quarter phase to be seen. Look for the star Spica in Virgo just to the south. In the third century BC, Aristarchus of Samos tried to measure the angle between the Sun and Moon at the time of the quarter. If the Sun were infinitely far away, the angle would be 90 degrees. He found an angle of 87 degrees, and concluded that the Sun was 20 times farther than the Moon. It turns out that the angle is impossibly small to find with the naked eye and he was off by a factor of 20 (the Sun 400 times farther than the Moon). But his brilliant conclusion that the Sun is much more distant than the Moon was indeed correct. Earlier in the week we see the Moon in the waning gibbous phase, later in the week it thins as a waning crescent. The morning of Friday the 16th, as Skylights week ends, the rising crescent glides just to the north of Saturn, the two of them above the star Antares in Scorpius.

As the week opens, the Moon is at apogee , where it is farthest from Earth. And because of the eccentricity of the Earth's orbit and the tilt of the terrestrial rotation axis, the latest sunrise took place around January 4, the latest dawn around the 8th. We will now see the morning skies ever so slowly getting lighter.

The evening sky features Venus, which is slowly breaking out of dusk as it climbs higher and higher. This week features a close interplay between Venus and Mercury, which passes greatest eastern elongation (to the east of the Sun) on Wednesday the 14th. The evening of Saturday the 10th, the two will be under a degree apart (Mercury the fainter), allowing an excellent opportunity to see the first planet from the Sun, providing you have an unobstructed west-southwestern horizon. Shortly after Venus sets and evening twilight dims, Jupiter rises in the east still to the west of Regulus in Leo. Climbing high, the giant planet crosses the meridian to the south around 2 AM. An hour and a half later, Saturn (fainter than Jupiter, but still bright first magnitude) rises right off the tip of Scorpius's three-star head. Back in the evening, Mars continues to set at 8 PM near the Capricornus-Aquarius border. Then catch Comet Lovejoy, which this week zooms northward west of Orion and Taurus; binoculars will help.

In the early evening, the constellations of the Andromeda myth fill much of the northern sky. The most southerly of them, Cetus, the Sea Monster or Whale, floats with his head above celestial equator. Higher and a bit to the right is the flat triangle that makes Aries (the Ram), next upward the eponymous Triangulum, and then the focus of the story, Andromeda herself.

STAR OF THE WEEK: CHI CET (Chi Ceti, plus HD 11131). Half a degree southwest of fourth magnitude Baten Kaitos (Zeta Ceti), one of the stars that outlines the Sea Monster of the Andromeda myth, lies fifth magnitude (4.67, almost fourth itself) Chi Ceti. One immediately thinks, aha!, a naked-eye double. But no, Chi Ceti is fairly close to us, 75.2 light years (give or take just a half), whereas Zeta is three times as far, the alignment just a coincidence, as so often seen. A closer look, though, shows another, fainter star of seventh magnitude (6.75), HD 11131 of the Henry Draper spectral catalogue, just 3.2 minutes of arc to the west that is 74 light years away (with an uncertainty of 5). Also called Chi Ceti B, could it belong to the brighter star? Chi proper (HD 11171) is a mid-temperature (an uncertain 6665 Kelvin) class F (F3) "giant" (but see below). With most of its light in the optical spectrum, Chi radiates at a rate of 5.7 Suns, which gives it a radius of 1.8 solar radii. Rotating at an equatorial speed of at least 61 kilometers per second, it makes a full turn in under 1.5 days. Theory gives it a mass of 1.4 Suns and shows it to be not a giant but a dwarf perhaps three-fourths of the way toward the end of its hydrogen-fusing lifetime of 2.8 billion years. The "companion," HD 11131, is an intriguing class G1 dwarf that bears a lot resemblance to the Sun. With a secure temperature of 5768 Kelvin, it radiates at a rate of just 0.87 Suns. Temperature and luminosity then give a radius of 0.94 solar. With a mass of perhaps 0.9 Suns, it appears to be a fairly young dwarf, and consistently, it's magnetically active. Are they a pair? Similar distances would make it seem so, and they are commonly listed as a "common proper motion" double, meaning that their movements across the sky relative to the Sun are identical, or nearly so. Were they at the same distance, they would be 4500 AU apart and take close to 200,000 years to make a full orbit. From Chi, its solar type neighbor would be some 20 times brighter than Venus appears in our skies, while Chi proper from the little one would be another seven times brighter. The error limits on the distance measures show that they could also be much farther apart and not a binary at all. Moreover, during the past 124 years, the stars' separation has increased by 9 seconds of arc, which is far too much for orbital movement even at their closest. So they are not a true binary. But that is not the end of the story. Chi Ceti is part of the "Ursa Major moving group" or "stream" that is related to Ursa Major Cluster, which consists of the five middle stars of the Big Dipper and then some. A moving group is not made of gravitationally bound stars, but is composed of stars that have a memory of their origins and move similarly, but are gradually separating as they orbit the center of the Galaxy. We might then speculate that Chi Ceti's nearby "sun" belongs to the UMa group as well and therefore keeps a minimal pace with Chi proper. Was our Sun part of one in the distant past? Quite likely, but at an age of nearly 5 billion years and having orbited the Galaxy more than 20 times, its mates are surely long lost.

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