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The Sky This Week, 2015 June 23 - 30

Follow the waxing Moon.
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Lunar crater Copernicus, imaged 2015 SEP 15

The Moon waxes in the evening sky this week, diving toward the southern reaches of the ecliptic as she brightens the midsummer night sky. Full Moon occurs on July 1st at 10:20 pm Eastern Daylight Time. This is the first of two Full Moons for the month; the next falls on the 31st. This Full Moon is popularly known as the Hay Moon, Buck Moon, or Thunder Moon. The latter seems particularly appropriate this year! Luna may be found just northwest of the first-magnitude star Spica on the evening of the 25th. On the 27th she is just two degrees northeast of the tongue-twisting second-magnitude star Zubenelgenubi. On the 28th look for Luna just a degree north of yellow-hued Saturn.

This is another week when the Moon dominates the night sky view. The summer Milky Way is washed out by Luna’s glow which is further scattered by the persistent haze of Washington’s famed heat and humidity. However, it is often this bank of stagnant air that provides us with some of our crispest views of our nearest neighbor in space. Air often forms an "inversion" over urban areas during hot muggy days, and the lack of currents in this layer often allows telescopes to perform at their peak resolving power. As the Moon waxes through her gibbous phases, a wide variety of landscapes present themselves for your enjoyment. During much of the week the terminator line creeps slowly across one of the largest impact features on Luna’s face, the Mare Imbrium, or Sea of Rains. This huge feature, some 1100 kilometers (700 miles) across, is one of the largest impact craters in the solar system. It was formed by the impact is a large asteroid about 3.9 billion years ago, and its floor was subsequently flooded by lava erupted from the Moon’s interior. It can be seen with the naked eye as the left-hand "eye" of the "Man in the Moon". It has many interesting features, the most notable being the so called "wrinkle ridges", where molten lava solidified into a sort of frozen wave. Its floor is littered with smaller impact craters. Just to the south of Imbrium is a prominent impact crater that’s best seen on the evenings of the 25th and 26th. This is the crater Copernicus, a 93 kilometer (56 mile) diameter feature that’s much younger than the Imbrium basin, excavated by a smaller asteroid about a billion years ago. As such it is one of the youngest geological formations on the Moon. Had it been funded, the proposed Apollo 20 mission would have landed on its floor!

The early evening hours hold the most interesting phenomenon for the week as the dazzling planet Venus catches up to and passes Jupiter. On the evening of the 30th the duo will be less than 1/3 of a degree apart for one of the closest planetary appulses of the year. Venus will sail a few degrees ahead of Jupiter over the next several weeks, but they will stay well within five degrees of each other through mid-July.

Jupiter now sets just after the end of evening twilight, but you still have a chance to get a glimpse of him as the sky darkens after sunset. You should still be able to view his four bright moons as twilight deepens, but you’ll need exceptionally steady air to see much detail on his distant cloud tops.

Saturn is hitting his stride as evening twilight ends, crossing the meridian after 10:30 pm. At his best he’s only 33 degrees above the southern horizon, so a sultry still night may give you your best view. From my yard he has to dodge trees for me to get a good view of him, but whenever I do catch a glimpse of him I still marvel at the sight he presents in the eyepiece.

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