August 24, 2013

What's Up in September: Signs of a season changing in the night sky


This month always marks the beginning of autumn for us in the northern hemisphere. That will happen at 4:44 p.m. on Sunday, Sept. 22. That moment is further defined by the sun on its ecliptic path crossing over the celestial equator on a downward path. The vernal and autumnal equinoxes are also the only two days each year that the sun rises due east and sets due west everywhere on Earth except at the poles.

Within a few days of those dates the length of the days are exactly 12 hours for everyone on Earth except at the poles. The reason is that we orbit the sun in ellipses and not perfect circles.

There will be many interesting new highlights this month as summer gradually fades into fall. This will include a great conjunction of Venus and Saturn, a minor meteor shower, keeping track of the exciting nova in Delphinius just below the Summer Triangle, and your first chance to get a look at Comet ISON right above Mars in the morning sky.

Venus continues to get closer and closer to Saturn in the evening sky at the rate of about 1 degree per day. On Sept. 8, the celestial pair will be just 10 degrees apart, which can be measured by one fist at arm's length. Look low in the west-southwestern sky 45 minutes after sunset and you will also see a slender waxing crescent moon gliding less than 1 degree below Venus with Spica, the brightest star in Virgo, just below the pair. The next evening the moon will already be 12 degrees farther east and closer to Saturn.

Brilliant Venus will pass just 4 degrees below the ringed jewel of our solar system during the evening of Sept. 18. After that the pair will drift farther apart again, but they will still form a nice, ever-changing conjunction along with Spica throughout the month. Notice that Venus is almost 100 times fainter than Saturn.

The minor meteor shower is called the Aurigids and it will peak on Sept. 1. Usually it only produces about six meteors per hour, which is just above the background rate of four stray meteors that you could see any night, depending on the darkness of your viewing location. However, as with most minor meteor showers, the Aurigids could suddenly produce a much greater number of meteors. This last happened in 2007, when it reached 130 meteors per hour. The moon will not interfere this year, so it will be worth checking it out and spending some quality time under the night sky around 4 a.m. on Sunday, Sept. 1.

You will be looking into a winter sky at that time, seeing Orion and the Winter Hexagon again in the eastern sky. You will also see Jupiter just above and to the right of Mars along with a waning crescent moon. Notice that Jupiter is about 30 times brighter than the red planet. After the meteor shower, keep watching as Mars drifts into the next constellation, Cancer the Crab. Mars will pass right through M44, also known as the Beehive cluster in Cancer on Sept. 8 and Sept. 9.

Try to get an early view of the next great comet, named ISON. It is following a very similar path to Mars all month long, only about 3 degrees above. Look for the comet early this month directly above the Beehive cluster before the moon gets too bright. You will need a good telescope to see it, since it will only be about 12th magnitude. It is expected to brighten to ninth or 10th magnitude by the end of the month. If it survives its close encounter with the sun in December, this comet could become really spectacular and possibly even visible in the daytime.

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