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The Sky This Week, 2012 June 12 - 19

Signs of the solstice

Saturn, as imaged with USNO's historic 12-inch telescope

The Moon continues to wane in the pre-dawn sky this week as she passes through the star-poor reaches of the rising autumnal constellations. New Moon occurs on the 19th at 11:02 am Eastern Daylight Time. The only bright object that Luna will cozy up to this week is the bright planet Jupiter, who will be just a degree west from the 5%-illuminated crescent in the gathering morning twilight of the 17th.

This week brings us into the roughly two-week "season" of summer solstice phenomena. The 13th marks the date of the earliest sunrise here in Washington. This will occur at 5:42 am EDT; by the week’s end sunrise will occur about a minute later. The latest sunset for the year will occur on the 27th. Set squarely between these dates is the solstice itself, June 20th, which will be the year’s longest day. The reason we have these apparent discrepancies results from the Earth’s elliptical orbit around the Sun. This causes our planet’s orbital velocity to vary slightly over the course of a year, while its rotation rate stays relatively constant. If we kept time using sundials there wouldn’t be an issue, but our punctual nature demands days defined by precise seconds. Astronomers thus use a "mean Sun" to describe the 24-hour day and apply a formula known as the Equation of Time to correct for Old Sol’s apparent positions in the sky. During the days around the solstices the Equation of Time goes through its most rapid changes in rate, so the various timings are spread out over several weeks. We’ll see a similar effect at the winter solstice in another six months.

Look to the western sky half an hour after sunset to catch a glimpse of the planet Mercury. This elusive world is making its second appearance in the evening sky this year, and while viewing circumstances aren’t as good as they were in his previous appearance in early March he should still be fairly easy to find. Mercury spends the week crossing the constellation of Gemini, so you can use the bright stars Castor and Pollux to help locate him. You’ll probably need binoculars to flush him out of the twilight glow as he climbs closer to the Twin Stars, but as the week progresses he should climb higher above the horizon haze.

Mars still draws attention in the southwestern mid-evening sky. His rosy tint sets him apart from the bright star Regulus in the constellation of Leo, and he is about half a magnitude brighter than the star. The red planet has moved from a position just five degrees from Regulus to a point where he’s now about halfway between the star and the bright duo of Saturn and the star Spica. Over the next few months he’ll close the gap on Saturn, passing the ringed planet in late August.

Saturn continues to be the best object to view in the night sky. You’ll find him nestled just five degrees to the north of the bright blue star Spica, and he’ll remain in close proximity to the star for the next several weeks. The planet’s rings are tilted 12.5 degrees to our line of sight, and the view of these otherworldly appendages through a telescope is always breathtaking. They are a sight that I have shown to countless people over the years, and they never fail to draw an amazed reaction.

If you’re up before the Sun take a moment to try to spot the giant planet Jupiter. He’s gradually emerging from the glare of solar conjunction, and soon he’ll begin attracting the attention of dedicated planet watchers eager to see the changes that have occurred in his turbulent clouds. He’ll be about a degree away from the slender crescent Moon before dawn on the 17th.