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The Sky This Week, 2012 June 26 - July 3

Hold on a second...
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The International Space Station glides over Saturn & Spica
Imaged from Alexandria, Virginia, 2012 June 25, 21:49 EDT


The Moon waxes in the evening sky this week, skirting the southern horizon as she waxes to Full phase on July 3rd at 2:52 pm Eastern Daylight Time. July’s Full Moon is popularly known as the Hay Moon, Buck Moon, or Thunder Moon. Look for the waxing gibbous near the bright star Spica and planet Saturn on the evening of the 27th. On the 30th she will be among the second-magnitude group of stars that form the "head" of Scorpius, the Scorpion, just a degree northeast of the star Dschubba. As July opens she wends her way into the heart of the Milky Way and the stars of the "teapot" asterism of Sagittarius.

Summer is officially a week old, and this week marks the end of the solstice "season". Our latest sunsets of the year are occurring on the first nights of the week, with Old Sol settling below the western horizon at 8:38 pm EDT here in Washington. It will probably take a couple of weeks before you really begin to notice that gradual shift of sunset to an earlier time, so enjoy the lazy evening hours for a while.

You’ll have an extra second to enjoy the evening of June 30th. The last minute of the day will feature the addition of a "leap second" to the timescale known as "Universal Time, Coordinated", or UTC, that regulates our "civil" time. This is necessary due to the nature of modern timekeeping, which is based on a second that’s defined by a resonant microwave frequency of Cesium atoms. This very precise frequency is stable over very long periods of time, but the Earth’s rotation, when compared to this timescale, is not. Since many people still like the quaint idea that "noon" occurs with the Sun is on the meridian, atomic time needs to be periodically adjusted to account for Earth’s sloppiness. At present the difference between 24 hours of atomic time and Earth rotation time is about two milliseconds. This difference, though seemingly very small, accumulates on a daily basis, so after a year or two the cumulative difference approaches one second. World timekeepers thus stop their atomic clocks for precisely one second at specified dates and times to let the Earth "catch up", thus "coordinating" the two timescales. Here in Washington the Leap Second will occur just before 8:00 pm, EDT, which is 00:00 UTC. What will you do with your extra second?

Elusive Mercury still flirts with the western horizon this week. The planet reaches greatest elongation east of the Sun on the evening of the 30th, but he is becoming more difficult to see as he fades by nearly half a magnitude. You’ll probably need binoculars to pull him out of the horizon haze in the evening twilight half an hour after sunset. At this time he’ll be about 10 degrees above the west-northwest horizon. As the week opens he can be found about 10 degrees to the left of Castor and Pollux, the Twin Stars of Gemini. By the end of the week he will have moved into the dim constellation of Cancer.

Mars now finds himself traversing the stars of the western part of the sprawling constellation of Virgo. On the evening of the 27th he’ll be just ¼-degree from the third magnitude stars Beta Virginis, which sports the wonderful Persian name of Zavijah. The pair should form a striking view in a small telescope, with Mars’ small ruddy disc contrasting nicely with the white pinpoint of the star.

Saturn spends the week seemingly suspended five degrees nort of the bright star Spica, the brightest star in Virgo. By mid-August Mars will pass the ringed planet and the star, but for now Saturn is the showpiece of the summer sky. His rings are tipped favorably in our direction, and the small phase angle between the Sun and the Earth cause the shadows of the rings on the planet and the planet on the rings to give a "3-D" effect when viewed with a modest telescope. Enjoy him now. In a few more weeks he’ll start to settle into the murky haze of the midsummer horizon.

Early risers should now be seeing two bright objects in the pre-dawn sky, Jupiter and Venus. Jupiter is about 15 degrees above the horizon at 5:00 am, while Venus is a few degrees below. If you use binoculars you may be able to sight the Pleiades star cluster just six degrees above the giant planet. Keep an eye on this part of the sky. There will be a spectacular gathering of the Moon, Venus, Jupiter, and the bright star Aldebaran is a few more weeks.

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