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The Sky This Week, 2013 June 11 - 18

Come see the stars on the National Mall!

 Saturn, Imaged at the U.S. Naval Observatory,
2013 June 5, 02:22 UT
Note Cassini's Division in the rings.

The Moon waxes in the evening sky this week, starting as a thin crescent in the southwestern sky and ending the week in a gibbous phase. First Quarter occurs on the 16th at 1:24 pm Eastern Daylight Time. Look for the bright star Regulus about seven degrees northwest of the Moon on the evening of the 14th. On the 18th Luna may be found just three degrees east of the bright star Spica.

As surely as day follows night summer will follow spring. This week marks the first milestone in the phenomena surrounding the summer solstice. Thanks to Earth’s elliptical orbit and our penchant for keeping atomic time we will experience the year’s earliest sunrise on the morning of June 14th. The year’s latest sunset will occur on the 28th. Sandwiched in between these dates is the solstice itself on the 21st, when the Sun reaches its northernmost point in the sky and we experience the year’s longest duration of daylight. These same phenomena occur around the time of the winter solstice as well, but with more pronounced effects. A detailed explanation may be found here.

Mark your calendars for the evening of June 14 (rain date June 15). If you are in the Washington, DC area you’ll have the chance to come down to the Washington Monument grounds to attend the third annual Astronomy Festival on the National Mall. This free event gets underway at 5:00 pm and lasts until 11:00 pm. Amateur astronomers from many of the region’s astronomy clubs will be on hand with a variety of telescopes to offer public viewing of the Sun, Moon, planets, stars, and brighter "deep-sky" objects. Representatives from NASA, JPL, the U.S. Naval Observatory, and various University astronomy programs will give brief talks and answer questions. This will be a great opportunity to learn about astronomy and the basics of observing for fun or more serious scientific purposes from the heart of the Nation’s Capital. If you can make it down for the evening, be sure to stop by the USNO table and say hello.

When we last looked at the early evening sky two weeks ago we were following the movements of three planets in the western twilight. Now there are only two. Bright Venus is the easiest one to spot, becoming visible shortly after sunset. Half an hour after sunset she should be very prominent about 10 degrees above the horizon. If you have binoculars look just a few degrees above and to the left of Venus to spot elusive Mercury. Once you’ve spotted him in binoculars you should be able to find him with the naked eye. Over the course of the week both planets will appear to stay "in formation", but by the week’s end Mercury will begin to fade and start a quick plunge back toward the Sun. The third planet in the trio, Jupiter, is now lost in the glare of the Sun. He’ll pass behind Old Sol next week and enter the morning sky.

By the end of evening twilight the warm yellow glow of Saturn stands on the meridian in the southern sky. The ringed planet doesn’t have the dazzle of Venus or the constantly-changing cloud belts of Jupiter, but he does have the solar system’s most amazing set of planetary rings. Thanks to the Voyager space probes we now know that all of the so-called "gas giant" planets sport these interesting appendages, but Saturn has the only set that can be routinely glimpsed with just a bit of optical aid. They are made up of billions of chunks of water ice, each in its own orbit around the planet’s hulking disc. Close-up images from the Cassini orbiter show amazing dynamics in the ring structure caused by gravitational effects from small embedded moonlets, and amateur telescopes will show a prominent dark band on nights of steady seeing. Known as Cassini’s Division, this band is a mostly-empty gap in the rings caused by a gravitational resonance with Saturn’s four innermost larger moons. To get a sense of scale when you see the Division through a telescope, consider that the width of the gap is about the same as the diameter of the Earth!

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