Skylights featured on Astronomy Picture of the Day

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Skylights featured three times on Earth Science Picture of the Day: 1 , 2 , 3 , 4 .


Photo of the Week.. Coming in from the gigantic Oort Cloud of comets that extends far beyond the planetary system, Comet Hyakutake of 1996 sports a delightful long gas tail caused by sunlight heating the icy comet's tiny nucleus and the solar wind blowing it backwards. The bright star at upper left is Arcturus in Bootes.

Astronomy news for the week starting Friday, February 18, 2005.

We begin, as always, with the wanderings of the Moon, which of course does not wander at all, but is under the strict gravitational control of the Earth and Sun. Meander might be a better word, since the orbit is not close to a simple ellipse, but is quite amazingly complex. Most of the week sees it waxing in its gibbous phase until it passes full the night of Wednesday, the 23rd, about midnight (as it crosses the meridian) in central Leo, to the east of the "Sickle," which makes the Lion's head, and the star Regulus.

Earlier in the week, the nights of Friday the 18th, Saturday the 19th, and Sunday the 20th, watch the Moon pass through central Gemini and above the planet Saturn, these two and Gemini's Castor and Pollux making a wonderful quartet. The next planet out, Uranus, passes conjunction with the Sun the night of Thursday, the 24th.

For all of ancient times, Saturn was the last of the planets, fainter than all the rest (though still quite bright) and with the longest period relative to the stars, 29.5 years. Then in 1781, with his telescope, William Herschel came across much dimmer Uranus, which is just barely visible to the unaided eye. Though seen before, Herschel was the first to make out its tiny disk. From the motion of Uranus (which, like all the other planets is gravitationally perturbed by its neighbors), Neptune was found in 1846. Finally, the "last planet," Pluto was discovered by Clyde Tombaugh at Lowell Observatory on February 18, 1930 (from photographs made on January 23 and 29), making this date the 75th anniversary of its discovery.

With its highly tilted orbit, Pluto (faint, requiring a decent telescope to see) is well off the ecliptic and is now in the morning sky in the constellation Serpens (the eastern part, Serpens Cauda). Averaging 40 Astronomical Units away, for a brief 20 years of its 248-year orbit, it actually comes closer to us than Neptune (30 AU away, the AU the average distance between Earth and Sun). Pluto is also gravitationally controlled by Neptune: averaged over a long period of time, Pluto goes around the Sun twice for every time Neptune goes around thrice. Planet or not? Pluto is really the largest body of the Kuiper Belt of small bodies that lies outside the orbit of Neptune (which supplies us with short-period comets). The stuff was too thinly spaced to assemble into a larger planet like Neptune or Uranus. There is no reason, however, why Pluto cannot be both a Kuiper Belt object and a planet at the same time.

Not of course to forget the others, very bright Jupiter (which now rises around 9:30 PM) and Mars, which rises in the southeast around 4 AM, an hour after Jupiter crosses the meridian.

Admire Orion, now crossing the meridian to the south around 8 PM. It is the top of a stack of constellations to the south of it that begins with Lepus (the Hare) and continues with the triangle that makes Columba (the Dove). Farther south and visible only from southern climes is Pictor (the Easel). Plunging farther down we find Dorado (the Swordfish), Mensa (the Table), finally the sky's south pole, which is surrounded by Octans (the Octant).
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