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 waning gibbous Moon, just 1.1 day past full, peeks through the trees.

Astronomy news for the two weeks starting Friday, October 7, 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.


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.

COMING IN OCTOBER!, From the Sun to the Stars by Jim Kaler, World Scientific 2016, a new book based on From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.

The next Skylightw will appear October 21, 2016.

The Moon brightens in the first part of our session, passing through first quarter the night of Saturday, October 8, around the time of moonset in North America. The waxing gibbous Moon then heads to full phase, when it is opposite the Sun the night of Saturday the 15th with the Moon climbing the eastern sky. The Moon then wanes in the gibbous phase, third quarter not reached until Saturday, October 22. The Moon passes perigee, where and when it is closest to Earth (about 5.5 percent closer than average) on Sunday the 16th, less than a day after full, the combination bringing especially high and low tides at the ocean shores.

The big lunar event is the occultation of the bright star Aldebaran in Taurus (the Moon crossing the star in its easterly motion) the night of Tuesday the 18th/morning of Wednesday the 19th, the exact time depending on location, but generally around midnight. The disappearance of the star will probably take a telescope to see as the leading edge of the gibbous Moon will be so bright. An hour or so later, Aldebaran will pop out the other side. The event is visible south of the line that connects Minneapolis, Denver, and Los Angeles. On the line itself an observer with a telescope can watch the star graze the edge of the Moon, flittering among crater edges.

The Moon will be to the right of Saturn the evening of Saturday the 5th, up and to the left the following night, Antares below. Mars has taken off for the east and Sagittarius. The Moon will pass north of the planet the nights of Friday the 7th and Saturday the 8th. In evening western twilight west you'll find Saturn and brilliant Venus. In the morning sky, Mercury lies close to Jupiter in bright twilight. At the end, Uranus is in opposition to the Sun on Saturday the 15th when it is just south of the nearly full Moon and quite invisible.

The Summer Triangle, made of three first magnitude stars and visible most anywhere, is in its full glory, with the bright star Vega in Lyra at the northeast apex, fainter Altair in Aquila at the southern, fainter-yet Deneb in Cygnus at the northwest apex. Far to the south the zodiacal constellations Sagittarius and Capricornus crawl along the horizon.

STAR OF THE WEEK: PI AQL (Pi Aquilae). "A close double star on the Eagle's northern wing, about 3 deg N. 1/2 W. from its lucida, A 6, pale white; B 7, greenish. This beautiful object may be termed a miniature of Castor" (Smythe and Chambers, speaking from 1881). The "lucida" is first magnitude Altair, the brightest star in Aquila, the Eagle, the numbers rounded-off magnitudes, which was about all astronomers could do with the visual estimates of the 1800s. The "greenish" color of the fainter, Pi Aquilae B, is an illusory contrast effect. The binary nature of Pi Aql, 514 light years away (give or take 42), was discovered by (who else?) William Herschel, who in 1783 also made the first measure of angular separation of 1.4 seconds of arc, the same as found in 2015, so the stars are keeping good pace with each other and are obviously gravitationally joined, though no significant orbital motion has yet been detected. There has been considerable confusion regarding the spectral classes and about which might be ascendant. The primary now seems to be a class F (F9) giant (once thought to be an F2 dwarf, even a G8 giant), while the secondary is most likely a class A3 dwarf and white, not "greenish." The problem is that the stars are so close together that the light of each contaminates that of the other, which makes evaluation of magnitudes difficult as well. We adopt visual magnitudes of 6.34 and 6.75, which together give a combined magnitude of 5.77, very close to that listed by others. A third suggested component, thirteenth magnitude Pi Aql C, 36 seconds of arc away, has shifted 4.4 seconds over the past 125 years, and can be dismissed as an optical alignment. There are no accurate temperature measures, so we adopt respective temperatures of 5900 and 8700 for Pi A and B, which with distance yields luminosities of 61 and 40 times that of the Sun and radii of 7.5 and 2.8 solar radii. Theory gives masses of 2.6 and 2.3 Suns, an age of some 600 million years, and shows that while the slightly lesser star is indeed a dwarf, the primary is more of a subgiant in cooling transit between dwarf and true gianthood, before it begins to make its first ascent brightening in preparation for the firing up of its now-dead helium core, thus explaining the similarity of properties of the two stars. At birth, they would have been seen as a sparkling pair of class B9 stars, one slightly brighter than the other. Given their angular separation and distance, they must be at least 220 Astronomical Units apart (the foreshortening not known) and take more than 1500 years to circuit one another. Given their minimum physical separation, from each the other would shine with the light of 500 full Moons. (If the primary is as cool as G8, 4900 K, then because of the addition of infrared radiation, its luminosity goes up to 76 Suns, the radius to 12 solar, and the mass to 3 Suns, theory then showing the star to be an actual helium-fusing giant, revealing how much there is to learn about the system.)

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