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. Jupiter, as seen through 26-inch refractor of the US Naval Observatory at 10:48 PM EDT on April 5, 2016, is covered with turbulent ammonia and hydrocarbon clouds. The southern hemisphere's obvious Great Red Spot, an anticyclone twice the size of Earth, has been diminishing over the past century or so. Image by Geoff Chester, with thanks.

Astronomy news for the two weeks starting Friday, May 6. 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.

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 Queen in Stellar Stories.

The next skylights will appear May 20.

Again the fortnight tracks the lunar phases. We start at new Moon on Friday, May 6, the thin crescent first becoming visible in western twilight the evening of Saturday, May 7. The evening of Sunday the 8th finds the waxing crescent to the right of Betelgeuse in Orion. The phase is terminated at first quarter on Friday the 13th (good luck to all) with the Moon beneath Regulus in Leo. The waxing gibbous Moon then finds itself southwest of Jupiter the night of Saturday the 14th, while the following evening it moves off to the east of the planet heading toward full phase on Saturday the 21st, when it will be to the left of Mars and above Saturn . Our period ends the previous night with the Moon to the northwest of the red planet. During the fortnight the Moon also moves slightly away from the Earth on its elliptical path, crossing apogee (farthest from Earth, 5.5 percent more distant than average) three days before full, which will weaken the high tides at the coasts.

Jupiter, high in the sky to the western side of the celestial meridian by the end of evening twilight, ceases retrograde (westward motion against the background stars) on Monday the 9th. The giant planet sets shortly before dawn. On the other side of the sky Mars rises in the southeast as twilight fades, followed half an hour later by Saturn, the two making a nice show with Antares in Scorpius.

The big event belongs to Mercury , which not only goes through inferior conjunction with the Sun on Monday the 9th, but also transits across the Sun. The transit begins at 6:12 AM CDT, when Mercury hits the eastern solar limb. Mid-transit just south of the solar disk is passed at 9:58 AM CDT and the event is over at 1:42 PM CDT when the Moon leaves the western solar limb. (Add an hour for EDT, subtract an hour for MDT, two hours for PDT). A properly-filtered telescope is needed to see little Mercury, which will appear as a small but very black dot just 10 seconds of arc across. Do NOT attempt viewing without a proper, commercially-made, solar filter. Home-made ones have been known to burst into flame. Projection is safest. Transits of Mercury are centered around November 9 and May 7 separated by intervals of 7 and 14 years, the November events twice as common. The next one will be November 11, 2019. Transits of Venus are much rarer. The last one took place in 2012 and we won't see another until 2117. The transits of Mercury have a long history in the establishment of longitude, as they make a natural clock with which to tell Greenwich time (which compared to local time yields east-west global position).

Though the whole constellation is not visible from mid-northern latitudes, the northern portion of Centaurus, the Centaur, crosses the meridian to the south in late evening. It's followed by the stars of Lupus, the Wolf, then by Scorpius, the Scorpion and by Sagittarius, the Archer, which is yet another Centaur, the mythological figure apparently quite popular among the ancients.

STAR OF THE WEEK: 1 AND 4 CEN (1 and 4 Centauri), another two-for one special. The stars of Centaurus (the Centaur) have an odd naming history. The letters look as if Bayer had done his usual job in his great atlas, the Uranometria, Greek letters first then lower then upper case Roman. However, he could not see most of the constellation, nor for that matter could Tycho Brahe, on whose observations the atlas is based. Instead the letters were re-applied using Greek then upper and lower case Roman (reversed from Bayer's scheme) by Nicolas de Lacaille, who explored the southern heavens. Working from England, Flamsteed could see no further down than 35 or so degrees south of the celestial equator, so he measured the positions of only four stars near the constellation's northern boundary with Hydra (the stars just south of Hydra's tail). These four were then later numbered from east to west. Of them 3 Centauri (actually 3 Cen A, a young B5 dwarf) is the best known, really famed, as it's one of the very few stars whose spectrum reveals the presence of the light isotope of helium, He-3 (of which there is practically none on Earth; all our helium comes from the decay of uranium and thorium and is He-4). 2 Cen is a fine red, slightly-variable, class M5.5 giant. So now we round off the quartet with 1 Centauri, a class F (F3) subgiant, and 4 Cen, classed a B4 subgiant but (see below) really a dwarf, all giving us a nice range of color. With a well-known temperature of 6790 Kelvin and distance of 63.8 light years (give or take just 0.3), 1 Cen radiates almost all its energy in the visual spectrum, shining with a luminosity of 6.0 Suns, which leads to a radius 1.8 times solar. A projected equatorial rotation velocity of 75 kilometers per second gives a rotation period under 1.2 days (the star just slightly warmer than the "rotation break" at F5 above which stars lose their outer convective layers and speed up; or rather don't slow down). Theory then reveals a mass of 1.4 Suns and shows the star indeed to be either a subgiant (whose core is exhausted of hydrogen fuel)or an older dwarf soon to become one, the age just under three billion years. By contrast, blue-white 4 Cen's temperature of about 16,400 Kelvin is not well-determined. Factoring in a lot of ultraviolet radiation and the distance of 637 light years (give or take 87), 4 Cen's total luminosity is around 1225 times that of the Sun, its radius 4.3 times solar. A rotation speed of at least 27 kilometers per second gives a rotation period less than 8.1 days. Since there are no obvious abundance anomalies caused by element separation in a quiet atmosphere, the star is probably rotating much faster (so as to keep things stirred up) with its rotation pole pointed more or less at us. With a mass of 5.5 Suns, 4 Cen appears to be an older dwarf closing in on its dwarf lifetime of 65 million years. Both 1 and 4 Cen have spectroscopically-detected companions with respective periods of 9.945 and 6.927 days, which, assuming they are of low mass, give from Kepler's laws a separation from their parent stars of 0.30 and 0.12 Astronomical Units. 1 Cen may be a Delta Scuti type of variable with a short period of 0.02 days, while 4 Cen has a line-of-sight 15th magnitude neighbor currently 15 seconds of arc away. Both 1 and 4 Cen will eventually slough off their outer layers and die as white dwarfs with respective masses of 0.6 and 0.9 times that of the Sun. If nothing else, the set of four stars shows the wondrous variety of the starry sky. (Thanks to Jerry Diekmann, who suggested these stars.)

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