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

Greenland 12

Photo of the Week. Planet Earth: the tenth of twelve in the "Flight across Greenland," going from east to west above the fantastic glacier and a river of ice. See full resolution.

Astronomy news for the week starting Friday, April 18, 2014.

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. Last week's Skylights is still available. 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 The Harp in Stellar Stories.

With the eclipse over, the Moon is seen at the beginning of the week late in its waning gibbous phase. That ends with third quarter on the night of Monday, April 21, about the time of Moonrise in North America. During the remainder of the week we see the Moon fading as a waning crescent, rising ever later in the morning hours as it approaches Venus. The morning of Friday the 25th, the two will make a fine sight with the rising Moon up and to the right of the brilliant planet. The Moon passes perigee, its closest point to the Earth (5.5 percent closer than average), on Monday the 22nd, less than a day after the third quarter.

Night after night, the planetary sky changes, albeit slowly. Now in the western sky as darkness falls, Jupiter sets shortly after local midnight. But you still have a couple months left to enjoy the bright planet, which quite overwhelms everything else. To the southeast, Mars is already well up by the end of dusk nicely to the northwest of Spica in Virgo, Mars's retrograde motion obvious over only a few nights. By midnight Daylight Time, the red planet is crossing the meridian to the south. Mars is followed by dimmer but still bright Saturn, which rises in the southeast near twilight's end still within the confines of Libra. The ringed planet then crosses to the south about 2:30 AM. Wait a couple hours and you get to see Venus popping up over the eastern horizon. The second planet from the Sun crosses a bit of a divide by rising just as morning twilight begins, no longer (for now) to be seen in a fully dark sky.

From the large to the small, we have a bit of a meteor shower this week, the Lyrids. The shower peaks the morning of Tuesday the 22nd, the meteors appearing to come out of the constellation Lyra. Rather sparse to start with, producing 10 or 20 meteors per hour, their number will be reduced by the bright quarter Moon. The Lyrids are the debris of long-period Comet Thatcher of 1861. It won't be back for some 260 years.

Leo, with its distinctive "Sickle" that ends in Regulus, crosses the southern meridian around 10 PM as we approach the days of summer. Following along behind it is Virgo with Spica. The two stars are closely connected by the ecliptic, the autumnal equinox falling roughly midway between them. To the west of Leo, between it and Jupiter-embracing Gemini, lies one of the dimmer constellations of the Zodiac, Cancer the Crab, which holds the Beehive star cluster and once held the Summer Solstice (hence the Tropic of Cancer). That it no longer has the solstice is the fault of precession, the 26,000 year wobble in the Earth's axis.

STAR OF THE WEEK: CHI CNC (Chi Cancri). Though only fifth magnitude (5.14), Chi Cancri nevertheless is part of the outline (at least that used here; others differ) of Cancer, the Crab, whose fame comes not from its bright stars but from its position in the ancient Zodiac, situated between Gemini and Leo (though its harboring of the Beehive cluster does not hurt). Don't confuse it with Chi Geminorum, which lies across the border in Gemini just 3.7 degrees to the west and is an ordinary orange K2 giant 256 light years away. Chi Cancri on the other hand is very different; as a class F (F6) hydrogen-fusing dwarf, it is not all that dissimilar from the Sun. And as a lower mass dwarf, it has to be much closer than Chi Gem, just 60 light years away (give or take under 1). From its point of view, our Sun would be a full magnitude fainter, mixed in with the stars of deep southern Capricornus. In spite of Chi Cnc's relative faintness, it's pretty popular. More than 20 temperature measures average 6270 Kelvin. With no significant correction for infrared or ultraviolet light, the star radiates at a rate 2.4 times that of the Sun, which gives it a radius of 1.30 times solar. Direct measure of angular diameter through interferometry yields a radius of 1.39 times that of the Sun, just seven percent higher, not all that bad. The mass is about 20 percent greater than that of the Sun, a more precise study giving 1.07 solar. From other studies, the age is close to that of our Sun, which because of its higher mass means that Chi Cnc is relatively older when compared with its hydrogen fusing lifetime of around 7.5 billion hears. Chi Cancri is just under the "rotation break" at class F5, hotter than which stars rotate much more rapidly as a result of a decline in their outer convective layers, which generate magnetic fields that, coupled with the stars' winds, slow them down. Chi Cnc rotates with a projected equatorial speed of just 5 kilometers per second, giving it a rotation period under 13 days. Yet the star seems to have no magnetic activity, which may have died away as a result of age. Bright dwarfs like Chi Cnc are prime targets for planet searches. Yet the star seems devoid. Especially telling, there is no evidence for any debris disk as seen around so many stars, including hotter ones like Vega. Perhaps that is related to the lower metal content, which is about half (relative to hydrogen) that of the Sun, planets rather liking higher values. The lower metal content may be related to a somewhat higher velocity through space relative to the Sun, about triple normal. Not only are there no planets (at least found), there are no companions either, the star quite alone in a lonely spot in the Zodiacal Crab.

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