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The Sky This Week, 2012 July 4 - 10

Pondering extremes

"Dragon Sunset", 2012 July 2, imaged near Morattico, Virginia. 

The Moon wanes in the morning sky this week, passing through the dim constellations of the autumn sky in the hours before sunrise. Last Quarter Moon occurs on the 10th at 9:48 pm Eastern Daylight Time. Luna finds little in the way of bright companionship this week. The best approach to a bright object will be at around 5:00 am EDT on the 8th, when the Moon is crossing the meridian about 27 degrees above the lonely first-magnitude star Fomalhaut. However, pre-dawn skywatchers will have a treat come the following week when the crescent Moon, Venus, Jupiter, the bright star Aldebaran and the Pleiades gather in the morning twilight.

It’s a bit hard to believe after our record heat wave of late, but the Earth is farthest from the Sun on the 4th at 11:32 EDT. At this time we will be 94,505,851 miles (152,092,424 kilometers) from Old Sol. Back on January 4 we were some 3,103,884 miles (4,995,217 kilometers) closer. As you can see, the variation in the aphelion and perihelion distances of the Earth are quite small compared to the overall mean distance to the Sun, but this quantity, known as the eccentricity of the Earth’s orbit, slowly changes over time. Eccentricity is expressed as a ratio, and currently it is about 0.017. It can vary between 0.005 (nearly circular) to 0.06 (about a 12% ellipse) with two superposed cycles of some 100,000 and 400,000 years. These variations are primarily influenced by the gravity of the giant planet Jupiter, and at times of extreme eccentricity this can have a major effect on the planet’s climate. Right now we’re trending toward a more circular pattern, though, so we can’t blame the current weather on Jupiter!

The warm and muggy evening hours still play host to three planets, but odds are you’ll only be able to track down two of them. Elusive Mercury still hangs in the western sky as the week opens, but he’s only five degrees up around 45 minutes after sunset. Shining at magnitude 1.0 he will be a very tough target to find without binoculars and almost impossible with the glasses unless the sky is very clear. He plunges from the sky this week as he races to pass Earth on his tight solar orbit. He will emerge in the morning sky for a good show in August.

Mars continues to press eastward through the stars of the constellation Virgo, with an eye set for Saturn. The red planet will pass just over a degree south of the third magnitude star Eta Virginis by the week’s end. There is a fifth magnitude stars, 13 Virginis, tucked close to eta, so when Mars comes calling this should be a very attractive sight in binoculars or a low-power telescope. Through much larger instruments Mars may still reveal a hint of detail, but the size of his apparent disc is now half of what it was at opposition back in early March.

Saturn is beginning a very slow eastward creep against the stars, moving a mere 10 arcminutes during the course of the week. You may still find him perched some five degrees above the bright blue stars Spica, and his motion relative to the star will be almost imperceptible until very late in his current apparition. Right now the ringed planet is also located in an interesting starfield for binocular observers, but the real treat is to see him through a telescope. Almost any instrument will show the planet’s icy rings, but for a good detailed view you’ll need a scope of at least six inches in aperture. This is actually a good time to observe Saturn; hazy hot evenings generally have stagnant layers of air overhead, so currents that can distort the view are often minimized. Just be sure to let your telescope warm up to the ambient air temperature and keep an eye open for thunderstorms!

The best planetary action (and the coolest temperatures) this week take place in the morning sky. Here you will find dazzling Venus transiting the Hyades star cluster just above the bright star Aldebaran, while Jupiter drifts between Venus and the Pleiades. You should have no trouble spotting the show about an hour before sunrise. While the view this week will be good, it will get even better next week when the crescent Moon closes in for a visit.

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