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The Sky This Week, 2010 June 29 - July 6

Earth at Aphelion, and an Evening Planet Horse Race
ThunderMoon_04_WP_small.jpg
Moon and Thunderhead, 2010 June 22
Imaged at the U.S. Naval Observatory with a
Canon PowerShot S2IS digital camera,
1/100s @ f/5.6, ISO 50

The Moon wanes in the morning skies this week as she gradually climbs back up from the southerly declinations of her midsummer path through the stars of Scorpius and Sagittarius.  Last Quarter occurs on July 4th at 10:35 am Eastern Daylight Time.  Luna spends most of the week among the dim starfields of the autumn sky except for the mornings of the 3rd and 4th, when she shares the pre-dawn limelight with the bright planet Jupiter.

July 6th marks the date of Earth’s aphelion, the planet’s most distant point from the Sun.  At 7:30 am EDT our fair world will be some 152,096,438 kilometers (94,508,345 miles) from the center of the Sun.  Fortunately for us the mean distance between Earth and the “day star” varies by a very small percent over the course of a year, which keeps our climate relatively benign.  Even over geological time-scales the eccentricity of Earth’s orbit varies by only about five percent, which is one of the reasons it has supported liquid water and an amazing variety of life forms for billions of years.

We are now beginning to see a very gradual decrease in the length of daylight as we are now well past the summer solstice.  By the end of the week night will have gained back some five minutes from day, and the time of sunset will begin to inch its way earlier each evening.  Most of us probably won’t notice the change in either parameter for another few weeks, so go ahead and enjoy summer’s long, lazy evenings.

The evening twilight sky plays host to the planetary equivalent of a steeplechase, with three planets vying for your attention.  Currently the brightest and first to appear is the ever-dazzling Venus, which becomes visible shortly after sunset.  Half an hour or so later you should be able to glimpse the ruddy tint of mars and the more serene yellow glow of Saturn.  The fun thing to watch this week (and over the next several as well) is the ever-tightening distance between all three worlds.  

Venus moves most rapidly against the background stars.  As the week opens she’s just over 10 degrees west of the bright star Regulus in the constellation of Leo, the Lion.  By the end of the week that gap shrinks to less than three degrees.  Through the telescope she resembles a dazzling white gibbous Moon.

By contrast, Mars is a tiny pinkish dot, one-third the apparent size of Venus.  He too shows a gibbous phase, but you’ll probably be hard-pressed to see much more detail than that.  The red planet is also on the move, marching steadily in the direction of Saturn.

Saturn is still the best target for the small telescope in the evening sky, but your best time to catch him is toward the end of evening twilight.  The planet’s famous rings are gradually opening to our line of sight, and in moments of steady seeing you should be able to trace their oval shape once again.  The ringed planet is also moving eastward against the stars, but his motion is virtually imperceptible to the naked eye.

Giant Jupiter will soon begin to grab our attention in the evening sky.  As July opens he rises at around 12:30 am EDT.  By the month’s end he’s up just after 10:30 pm.  Old Jove gets a visit from the Moon on the mornings of the 3rd and 4th this week, otherwise he’s all alone in a very star-poor region of the sky.  He does have one unusual faint companion, though.  Some two degrees to the west is the distant planet Uranus, which will resemble a sixth magnitude greenish “star” that resolves into a tiny disc under high magnification.  The two planets will remain fairly close to each other throughout the late summer and fall.

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