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The Sky This Week, 2011 July 5 - 12

A long-awaited "anniversary"...
Neptune, imaged 2009 SEP 20
at the U.S. Naval Observatory

The Moon waxes in the evening sky this week, passing from the constellations of late spring into those of early summer.  First Quarter occurs on July 8th at 2:29 am Eastern Daylight Time.  Luna passes about eight degrees south of Saturn on the evening of the 7th.  The following night finds her four degrees southeast of the bright star Spica.  On the 11th look for the Moon just two degrees above the ruddy star Antares low in the southern sky as the midnight hour approaches. 

In addition to the 235th anniversary of American Independence, July 4th marked another milestone in the calendar.  At 10:54 am EDT the earth passed aphelion, its farthest point from the Sun.  At this time the center of the planet was 152,102,140 kilometers (94,511,888 miles) from the center of the Sun.  For the next six months we’ll gradually inch back toward the day star, reaching perihelion on January 4th, 2012.

The end of the week marks an interesting historical “anniversary” of sorts.  On July 12th the far-flung planet Neptune will complete its first full orbit of the Sun since its extraordinary discovery on the night of September 23rd, 1846.  On that crisp autumn evening nearly 165 years ago two German astronomers, Johann Galle and Heinrich d’Arrest, pointed the 9-inch refractor at the Berlin Observatory to a patch of sky in the constellation Aquarius where the French mathematician Urbain J.J. Le Verrier predicted that they would find a planet.  Using a newly-compiled but unchecked star atlas the astronomers quickly found a “star” that was not on the map about one degree away from the place where the new planet was predicted to be.  Careful examination showed the “star” to have a tiny pale blue disc, and thus Neptune was revealed.  Le Verrier’s prediction was based on irregular motions of the planet Uranus, and similar reasoning by the English mathematician John Couch Adams led to a similar solution.  Neither Adams nor Le Verrier could muster much interest in the search for the planet in their own countries; Le Verrier’s letter to the director of the Berlin Observatory ultimately led to the find.  While Galle and d’Arrest are given credit for Neptune’s discovery, they were not the first people to see it.  In December of 1612 and January of 1613 none other than the famous Galileo recorded it as a faint background star near Jupiter, but his crude telescope couldn’t show Neptune’s slow movement against the background stars. 

When Neptune was found on that night in 1846, it was quite close to Saturn in the sky.  Galle and d’Arrest must surely have looked at it during the course of the evening, and its appearance would have been similar to the view we currently are enjoying.  The ringed planet is almost halfway around the sky from its location in 1846, though, having completed about 5.6 orbits of the Sun to Neptune’s one.  Saturn is now best seen in deepening evening twilight in the southwestern part of the sky.  It is easily found as the westernmost apex of a triangle formed by itself and the bright stars Arcturus and Spica.  A modest telescope of 8-inch aperture should readily duplicate the planet’s appearance as it may have been seen by the bygone German astronomers as it floats against distant stars accompanied by a bevy of tiny moons. 

Giant Jupiter now rises shortly before 2:00 am, and by 5:00 he is well up in the southeast.  Early risers are once again beginning to focus telescopes on his turbulent cloud belts, and so far the trends reported at the close of last year’s apparition are continuing.  The South Equatorial Belt, one of Old Jove’s most prominent dark features, has returned from last year’s absence and should be easily visible in small telescopes.  Not being a “morning person” I have yet to image the planet myself, but if we get a spate of good weather I may have to haul the scope out for a pre-dawn look.  

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