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Minnesota State University, Mankato
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July brings classic summer sky, with largest full moon, aphelion

Skies are classic in July, says MSU Astronomy Professor Steve Kipp, with the largest full moon of the year, and a July 4th NASA rocket strike on the comet Tempel 1.

By Dr. Steve Kipp, MSU Professor of Astronomy

The classic summer sky is on display in July.

When the sky is completely dark, the constellation Scorpius the Scorpion is found curled up low in the south. The pattern of the Scorpion's body, marked by the bright red star Antares, with a curved tail, is particularly easy to recognize. To the left of Scorpius is Sagittarius the Archer, whose bright stars make the shape of a teapot with a top, handle and spout. The white band of the Milky Way goes directly through Sagittarius. In fact, when you look at the Milky Way in Sagittarius, you are looking towards the center of the disk of stars that makes up our Galaxy.

The star patterns are fixed, but the planets and the moon move through the stars. This July at sunset, Mercury and Venus are still close together, low in the western sky at sunset. On July 8, they will be joined by a crescent moon. Bright Jupiter may be seen high in the southwest, even before the sky is completely dark. On July 17, the waxing gibbous moon will be about one apparent moon diameter from Antares in Scorpius.

NASA plans to celebrate the 4th of July this year by impacting a probe onto the nucleus of comet Tempel 1. The Impactor, about three feet across and weighing about 800 pounds, will be launched from NASA's Deep Impact Spacecraft. The nucleus of Comet Tempel 1 is about four miles across and is probably made of ices and bits of rock. The Impactor will hit the comet nucleus at a speed of 23,000 mph. NASA thinks the impact will make a crater that might be as large as a stadium. Analysis of the impact will help reveal the composition of the comet nucleus.

This event is scheduled to happen about 1 a.m. CDT, July 4th. At that time, Comet Tempel 1 will be low on the western horizon for observers in southern Minnesota, but only visible with a telescope. The impact will make the comet much brighter for a time, but not bright enough to see with the naked eye. To learn more about this exciting event, go to the following NASA website:

At the start of July 5th, Earth reaches aphelion, its greatest distance from the sun. Aphelion is 94,512,050 miles, not much larger than the Earth's average distance of 92,955,807 miles. The Earth's orbit is nearly circular.

July's full moon, on the 21st, is the largest of the year. This is because the full moon occurs when the moon is at perigee, its close approach to the Earth. At full moon, the ocean tides are extreme because the moon and sun are lined up and pulling together.

Near the end of July, meteors will compete with fireflies. The south delta Aquarid meteors peak July 27. Unfortunately, the last quarter moon will hide fainter meteors. Look towards the east after midnight. If you observe a few days after the peak of the meteor shower the moon will be less of a problem.

Dr. Steve Kipp is a professor of astronomy at Minnesota State University, Mankato.  Questions or comments may be sent to MSU, 141 Trafton Science Center N,  Mankato MN, 56001 or send E-mail to

STARLINE is Minnesota State University's Monthly Astronomy Information Service

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