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The Sky This Week, 2010 June 15 - 22

An evening planet traffic jam, and summer's here!
No Observing Tonight!
An afternoon thundershower passes just south of the USNO
on 2010 June 1

The Moon waxes in the evening sky this week.  She starts off in the vicinity of bright Venus on the 15th, passes below fading Mars on the 16th and 17th, then passes eight degrees south of golden Saturn on the 18th.  First Quarter occurs on the 19th at 12:29 Eastern Daylight Time.  By the week’s end Luna is diving down to encounter the rising constellations of summer.  

The summer solstice occurs on June 21st at 7:28 am EDT.  At this moment the center of the Sun’s disc stands directly overhead at a point on the Tropic of Cancer in the southeastern corner of Algeria.  A few hours earlier Old Sol stood virtually overhead in the Egyptian city of Aswan, known to the Greek astronomer Eratosthenes in about 240 BCE as Syene.  Eratosthenes also knew that on the summer solstice in Alexandria, Egypt the Sun was just over seven degrees from the zenith.  Having traveled by camel from Syene to Alexandria, he estimated the distance between the two cities and used geometry to estimate the circumference of the Earth.  His result was remarkably close to our modern value if we make certain assumptions about the units that he used.  Still, his method proved that the Earth was indeed a sphere of finite dimension.  For most of us, the solstice passes more or less unnoticed except as the marker of the longest day of the year.  However, many ancient cultures revered the day, as evidenced by Neolithic and Paleoamerican sites throughout the world.

Venus continues to grace the western evening sky, slowly dipping toward the west-northwest horizon as evening twilight fades to darkness.  The dazzling planet hosts the waxing crescent Moon as the week opens, then passes just north of the binocular-target Beehive star cluster on the evenings of the 19th and 20th.  Through a small telescope her disc resembles a gibbous Moon with a dazzling white, generally featureless texture.  You’re looking at a perpetual pall of clouds which shroud the planet’s surface, hiding the hellish conditions of a runaway “greenhouse effect” from the carbon dioxide trapped under the cloud decks.

Ruddy Mars spends the week moving eastward from the star Regulus on his way toward an encounter with Saturn.  If you have binoculars you can watch him pass within a degree of the 4th magnitude star Rho Leonis on the evenings of the 18th and 19th

Yellow-tinted Saturn is now beginning to inch eastward against the faint stars just north of the third magnitude star Zavijava in eastern Virgo.  He crawls about one eighth of a degree during the course of the week.  By this time next year he will have moved just over 13 degrees, passing close to the close double star known as Porrima.  The planet’s famous rings are also beginning to open up to our line of sight, but they will still appear nearly edge-on in most small backyard telescopes.

Giant Jupiter is gradually making progress toward the evening sky.  By the end of the week Old Jove rises at around 1:30 am, and he is well up in the southeastern sky as the first rays of twilight begin to brighten the eastern horizon.  Jupiter is very hard to miss with the naked eye as he sits in one of the most barren starfields in the heavens.  He is also a treat for owners of almost any sized telescope.  The four large moons discovered by Galileo 400 years ago constantly dance around the planet’s striped disc, which is still missing the prominent South Equatorial Belt.