SKYLIGHTS

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

Mars and Saturn

Photo of the Week.. From top to bottom: Saturn, Mars, and Antares, all in a fine row on August 24, 2016. See full resolution. Compare with the positions on June 5 and July 8, when Mars was far to the west.


Astronomy news for the two weeks starting Friday, November 18, 2016.

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.

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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, now in Chinese translation.

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.


SSLive in town? Read FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. See the interview with Jim Kaler.


NEW! Read Dust to Dust in Stellar Stories.

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



The next skylights will appear December 2, 2016.

The Moon starts off in the early waning gibbous phase, as it heads toward last quarter the night of Sunday, November 20, the perfect phase actually achieved in the early morning of Monday the 21st a few hours after moonrise in North America. It will then make a pretty sight southeast of Regulus in Leo. The waning crescent thereafter plows through Virgo, coming close to Porrima (Gamma Virginis) on the morning of Tuesday the 24th, with bright Jupiter below them. By the morning of Friday the 25th, Porrima, Jupiter, the Moon, and Spica will fall along a ragged line pointing down toward the horizon. The moon finally passes new moon on Tuesday the 29th. Your last view of the ultrathin crescent will be in bright eastern twilight the morning of Monday the 28th. The crescent passes apogee, where it is farthest from the Earth, on Sunday the 27th.

The Moon then flips to the other side of the sky, the thin waxing crescent first visible in twilight the evening of Wednesday the 30th. Look for Venus (you don't really have to look for it) up and to the left. The two will make an especially fine pair the morning of Saturday, December 3. Up and to the left of them both, find Mars.

As to the planets, the sky gives us brilliant Venus, which now sets well after the end of twilight, and much fainter Mars, which sets at its reliable 9:30 PM as it climbs northward through Capricornus. Saturn is lost to bright western twilight. In the morning, Jupiter is up by 2:30 AM.

Look to the west for orange Arcturus, the brightest star of the northern hemisphere. Northeast of it is the beautifully curved set of stars called Corona Borealis, the Northern Crown. Its counterpart in the south, Corona Australis, is lost to twilight. Below the Northern Crown is a dim "X-shaped figure that represents the head of Serpens, the Serpent that wraps itself around Ophiuchus.

STAR OF THE WEEK: GAMMA CRA (Gamma Coronae Australis). The modern constellation of Corona Australis, the Southern Crown, is fainter than its northern counterpart, Corona Borealis, but is nevertheless easy to find due south of the Little Milk Dipper of Sagittarius and east of the tail of Scorpius. The top brightest stars, Alpha through Epsilon, are a bit unusual as they actually line right up in order of brightness, Alpha (the luminary at 4.11, tied with Beta), then fourth magnitude (4.35) Gamma, followed almost immediately by Delta and Epsilon. Bayer was supposed to have worked this way, but often did not. Of the five (and there are many fainter ones that complete the constellation), Gamma is just barely the most northerly of them. At first it looks like a bit of a cheat, since Gamma is not one star, but two, an orbiting binary made of two mid-temperature class F (F8) dwarfs of magnitudes 4.53 and 6.42. Even removing the companion, however, renders Gamma CrA in third place. Not just a binary, it's a visual double whose first measure of separation was made by John Herschel (son of William) in 1834, who called it "superb." The stars are currently 1.4 seconds of arc apart. Early on they were suspected of orbital movement, and by now we have a complete observed orbit wherein the stars go around each other with a period of 121.76 years. The distance to the binary was not observed by Hipparcos, so we must use alternative measures.

Gamma Cra With a well-defined orbit, Gamma Coronae Australis B goes around Gamma A (at the cross) every 122 years (in reality both orbiting a common center of mass roughly halfway between them) averaging some 33 AU apart. Unfortunately the distance to the system is not all that well known, so the results are a bit problematic. (From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.)

An uncertain distance of 56 light years (give or take 0.8?) yields a mean separation of 32.8 Astronomical Units, a fairly high eccentricity bringing them as close together as 22 AU and as far apart as 43 AU. They were last physically closest (though not necessarily closest as seen on the sky) in mid-2000 The classes are a bit problematic, as the fainter has been listed as cool as G1, similar to out G2 Sun. Kepler's Laws applied to the orbital parameters gives a combined mass of 2.4 times that of the Sun. With a temperature of 6188 Kelvin and thus little infrared radiation to add to the starlight, Gamma CrA A has a luminosity about 3.7 times that of the Sun, which gives it a radius of 1.7 times solar, and a mass near 1.25 Suns. Gamma CrA B, however, can't really be analyzed as it can't be fitted to theory. The problem seems that the class and temerature are just not known. If we simply say that it too carries 1 1/4 solar masses, then we come up with a sum of 2.5 Suns, almost exactly that derived from the orbit. If The projected equatorial rotation speed is 7.6 kilometers per second, Gamma CrA A rotates with a period less than 11 days. Whatever the details, both stars are beneath the "rotation break" near class F5, above which (hotter than) main sequence stars spin faster as a result of the loss of their outer convection zones and magnetic fields, the latter acting to slow the stars down as they are dragged out by the stellar wind. The stars ae too far apart to affect each other during later evolution, and the system will probably die as a double white dwarf.