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.Cloud shadows create a stunning sunburst.

Astronomy news for the two weeks starting Friday, December 19, 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 Under the Handle in Stellar Stories.

The next Skylights will appear Friday, January 2, 2015. Thanks for your patience. Best wishes for a merry and happy holiday season.

We begin with the Moon as a thin waning crescent visible in morning twilight as it approaches new on Sunday, December 21, in celebration of the passage of the Sun over the winter solstice at 5:03 PM CST (6:03 EST, 4:03 MST, 3:03 PST), which takes place the same day. Winter then officially begins as the Sun bottoms out at its extreme southerly position in Sagittarius, 23.4 degrees below the celestial equator, giving minimum heat to the northern hemisphere as well as the shortest day and longest night. The Sun will thereafter begin climbing back north toward the Vernal Equinox and the first day of spring. The morning of Saturday the 20th, the crescent will appear down and to the left of Saturn.

After new phase, the Moon advances in the evening as a waxing crescent, appearing nicely in western twilight. Look for it above Venus the evening of Tuesday the 23rd, more of less between the bright planet and Mars. The Moon appears to the right of Mars Christmas Eve, when the Moon also passes perigee, its point closest to Earth. Christmas night, the growing crescent will be up and a bit to the left of Mars. The waxing crescent terminates at first quarter on Sunday the 28th about the time of its daylight rise in North America, and it then enters the waxing gibbous phase, full Moon not reached until January 4, 2015. The Moon passes north of Neptune on Friday the 26th and just north of Uranus on Sunday the 28th, the planet ceasing retrograde motion on Monday the 22nd.

It's all about Jupiter now, the giant planet rising brightly in far western Leo to the west of Regulus just before 9 PM at the beginning of our session, an hour earlier by the end of it, Jupiter crossing the meridian to the south between 3 and 4 AM. But watch out, as Venus is coming up to rival it, our planetary neighbor now appearing low in southwestern twilight. In between we have Mars, which, still first magnitude, is good until the usual 8 PM as it treks through Capricornus. Shortly after Jupiter transits to the south and about an hour before dawn, Saturn rises just to the northwest of Scorpius's three-star head.

While some of the summer stars, including Cygnus and Lyra (with bright Deneb and Vega), are still with us early-on in the northwest, the fall stars, including Pegasus with its Great Square and the rest of the Andromeda gang, dominate the early evening as we await the stars of winter, mighty Orion rising as twilight fades. Look also in the northwest for bright Capella in the pentagon-shaped Auriga, the Charioteer.

STARS OF THE WEEK: CHI AND PSI PEG (Chi and Psi Pegasi), another two-for-one special. And this one is a natural for its rather remarkable symmetries. As Bayer neared the end of the Greek alphabet in his star-naming scheme for Pegasus (the Flying Horse), he arrived at Chi and Psi Pegasi, which straddle the eastern side of the Great Square (Alpheratz to Algenib), Chi to the south and east, Psi to the north and west, the two separated by six degrees (Chi almost on the Pisces border and notably closer to the Square's outline). The stars also straddle the equinoctial colure, the great circle that connects the equinoxes and the celestial poles, with Psi now the closer one of the pair to the colure. What is most remarkable is that that the two fifth magnitude stars (respectively 4.80 and 4.66) are both rather rare but quite similar class M (M2 and M3) red giants that appear to be in the same state of evolution. Chi is 368 light years away (give or take 9), Psi 476 (plus/minus 17), so they have no physical relation. With cool temperatures of roughly 3860 and 3590 Kelvin, Chi and Psi shine with the respective light (including a lot of infrared) of 440 and 1470 Suns, have radii of 47 and 100 times solar (if placed at the Sun would stretch halfway to Earth), and rather uncertain masses of around 2.5 to 3 Suns. The best guess is that both are brightening as giants for the second time with dead carbon/oxygen cores (the first brightening is with a dead helium core). Both are slight variables of a few hundredths of a magnitude and uncertain periods. As the more luminous, Psi appears the more advanced towards final expiration, when it will lose its outer envelope through a powerful wind (which nobody really understands), expose its hot inner core (which will light up the lost mass as a planetary nebula), and turn into a white dwarf of around 0.7 solar masses. Chi will soon follow. Now they diverge. Chi is moving at a fairly high pace relative to the Sun, 67 kilometers per second, over four times normal, whereas Psi is moving more leisurely at half that pace and in a different direction, emphasizing the stars' independence. Chi appears single, whereas Psi is binary.

Psi Peg Only a few observations determine the orbit of Psi Pegasi B about Psi Peg A (at the cross). In reality, each star is in orbit about a common center of mass that cannot be defined for this pair because of insufficient data. Given a distance of 476 light years, the fitted path suggests a separation of 25 Astronomical Units and a period of 55.1 years. Psi A does not appear at the focus of the fitted ellipse because of the tilt and orientation of the orbit, which does not appear face-on. (From the US Naval Observatory Double Star Catalog, courtesy of Bill Hartkopf.)

Its companion, of unknown nature but presumably small, is in a 55.1 year orbit with an average separation of 25 Astronomical Units, a modest eccentricity of 0.1 taking them between 22 and 27 AU apart. They were last physically closest in 2001 and will be again in 2056. Kepler's laws give a combined mass of 5.0 solar, a bit high, no surprise given a separation of just a couple tenths of a second of arc. Precession, the 26,000-year wobble in the Earth's axis caused by the gravitational pull of the Moon and Sun on our planet's rotational bulge, is dragging the Vernal Equinox to the west, which steadily increases the "right ascensions" of stars in this part of the sky (right ascension similar to longitude on Earth). The colure passed Chi Peg in 1718, making the star's right ascension jump from the 23-24 hour range back to 0 hours, the star thus beginning its cycle all over again. The colure will cross Psi in 2043. Party time.

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