Jimmy Westlake: September’s celestial highlights
August 31, 2010
Steamboat Springs — September is a month of change as we move from summer into fall. In Steamboat, this transition officially takes place at 9:09 p.m. Sept. 22. That's the moment the sun crosses the equator heading south for its six-month hiatus from the northern hemisphere.
This autumnal equinox is one of only two days each year when every location on Earth gets 12 hours of sunlight and 12 hours of darkness. The other day is the spring equinox in March. You've probably already noticed that the long days of summer are getting shorter and, after the equinox, this shortening will continue as we move toward the winter solstice in December.
The full moon that falls closest to the September equinox traditionally is called the Harvest Moon. Watch for it rising in the east at sunset on Sept. 23, just one day after the equinox.
The evening planets that we've enjoyed for most of summer will hang around after sunset for a little while longer. Saturn is nearly lost in the sunset glow as September begins and will pass behind the sun and into our morning sky on Sept. 30. Watch for a beautiful grouping of the crescent moon with the dazzling "evening star" Venus, the much fainter and redder planet Mars, and the bright star Spica on the evening of Sept. 10 and again Sept. 11. Look low in the southwestern sky between 8 and 8:30 p.m. Enjoy this conjunction because by October, our evening planet parade will be over.
Then, on the night of the equinox, watch for the brilliant planet Jupiter to rise just below that big old Harvest Moon and follow it across the sky all night until sunrise. Jupiter reaches its full phase as viewed from Earth on Sept. 21. That is the night of its opposition to the sun and its closest approach to Earth for the year, 368 million miles away. Just by coincidence, the more distant planet Uranus reaches its opposition on the same night as Jupiter, but much farther away at 1,780 million miles from Earth. The two planets will be visible together in binoculars less than 1 degree apart for about a week on either side of Sept. 21. Uranus will look like a faint little star just above dazzling Jupiter.
Early risers in mid-September might get to glimpse the illusive planet Mercury right before sunrise. Mercury reaches its greatest elongation west of the sun Sept. 19. Look due east at about 6 a.m. for a lone star shining through the morning twilight.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all across the world. His "Celestial News" column appears weekly in the Steamboat Today newspaper and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Also, check out Jimmy's astrophotography website at http://www.jwestlake.com.