(To use this Star Chart, face the direction indicated on the edge of the map
and hold it slightly elevated if front of your face)
This month’s night sky is dominated by the giant planet Jupiter. Jupiter is easy to pick out in the early evening. It is the very bright star-like object about halfway up the sky to the south. If you have a pair of binoculars and a steady hand, you can easily see a few of its moons, as tiny points of light to either side of it.
Jupiter is the largest planet in our Solar System and in fact is larger than all the other planets combined. Its diameter is about ten times Earth’s (think of a raisin and a soft-ball).
Depending on the time and day that you look, the Moon too may be visible in the night sky. Early in October, the Moon rises in the east a little before sunset. This month’s full moon occurs on the 3rd and is known as the Hunter’s Moon. The Moon’s orbital motion around the Earth makes it rise later each night and, by mid-October, the Moon will no longer be visible in the evening sky but will be visible in the dawn sky. By October 20, the Moon will be back in the evening sky, low in the west at sunset.
Before dawn on October 21, you may see a few “shooting stars” or meteors, part of the annual Orionid Meteor shower. On an average night, you can see a shooting star about every 20 minutes. On the 21st, you may see one roughly every 5 minutes. The Orionid meteor shower occurs when the Earth crosses the orbit of Halley’s Comet. At that time, dust particles left by the comet along its path may fall into our atmosphere and burn up. (Halley’s Comet itself is in the far outer parts of the Solar System at this time).
October’s night sky holds many famous constellations. Looking north, you can see the group of stars known as the Big Dipper. These seven stars are part of the Constellation Ursa Major, the Big Bear. The two stars that form the lip of the dipper point to the North Star, Polaris, a relatively dim star. Polaris is special to us here on Earth because if you stood at the Earth’s north pole, Polaris would be directly overhead. From other spots on Earth, the direction to Polaris marks almost exactly true north.
Here in southern Arizona, other stars are overhead, but which they are depends on the time of night and the season of the year. In October, a trio of bright stars is overhead in the early evenings. These three stars, separated by about a spread hand at arm’s length, are Deneb, Vega, and Altair. All three look bluish and Vega looks brightest. It lies in the constellation Lyra, the Harp. Deneb is in reality a brighter star than Vega, but it is much farther away and so it looks slightly dimmer to us. It marks the tail of the Swan in the constellation Cygnus. Cygnus is also known as the Northern Cross, with Deneb marking the top of the cross. Altair is in the constellation Aquila, the Eagle. It is the dimmest of these three stars and can be identified by the two slightly even dimmer stars on either side of it. You can find a detailed map showing these three stars (known as the Summer Triangle) on the web at http://en.wikipedia.org/wiki/Summer_Triangle.
Toward the northeast in the early evening you may notice a “W” shaped constellation. Its five stars form the chair in the constellation Cassiopeia (said Cass-ee-oh–pee–uh, with the accent on the pee).
Toward the south, you may be able to make out the constellation Sagittarius. It looks a little like a teapot, with handle, lid, and spout. The teapot even looks like it is steaming, but the pale glow around it is the Milky Way, not steam. With a pair of binocular you can see that the Milky Way is really millions of stars whose individual light is too dim for our eyes to make out, but which when added together, we can see as a pale glow.
The star chart may help you find some of these constellations. To use it, face the direction indicated on the edge of the map, and hold it slightly elevated in front of you. The map covers the whole dome of the sky, so the Big Dipper, for example, covers an area on the sky about the size of your spread hand at arm’s length. You can find many similar maps on the web.
Tom Arny, a resident of Patagonia, taught astronomy at the University of Massachusetts in Amherst, MA for many years. He did his graduate studies at U of Arizona and is very glad to be back again to this amazing part of our planet.
He will be writing a monthly column on a regular schedule
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