The Night Sky for September 2009
The Fall Equinox, Saturn's Rings Disappear, and a Europa-Io Eclipse
Aug 20, 2009
The sun provides the telltale sign that fall is coming in the Northern Hemisphere. Watch out the window each night and observe where the sun sets. The location will change each night, moving quickly toward the south.
The First Day of Fall for 2009
The September equinox brings the first day of fall for residents of the Northern Hemisphere. The official start of fall occurs at 5:18 p.m. EDT on Tuesday, September 22. The astronomical start of fall means that Earth is no longer tilting the northern half of the globe toward the sun. Instead, the sun is directly over the equator. It has moved from a more northerly location in the sky and will be heading south for the winter.
The equinox is often referred to as a time when the day and night is equal. This is not precisely the case. Day and night are quite close to being equal, more so than in summer or winter, but it would be more accurate to look at it as if the length of time for night in both hemispheres is equal.
Saturn Loses Its Rings
The Ringed Planet has been tilting with respect to the sun so that the rings have been reflecting less light toward Earth. The sight of Saturn's shrinking rings has been captivating observers all summer. Unfortunately, now that the big moment has arrived, Saturn is too close to the sun for good viewing. September 4 is the date when Saturn's rings are edgewise to Earth. Saturn makes for poor viewing all month, reaching conjunction with the sun on September 17. In October Saturn will move into the morning sky and its rings will begin tilting enough to provide a view of their northern face.
Jupiter's Moons Occult and Eclipse One Another
Telescope users will have a few chances to catch Jupiter's moons Io and Europa eclipsing and occulting one another. These two moons are the closest to the planet Jupiter. Jupiter is easy to find, shining brightly at magnitude -2.8 in the south, the brightest starlike point in this part of the sky after dark. On three Tuesday nights in September, the satellite Io will first occult (pass in front of) Europa and then as it moves away it will eclipse the other moon (cast its shadow on Europa).
On September 15 Io occults Europa starting at 8:42 p.m. EDT. This timing means it will be too bright in the other time zones of the United States to see the event, which only lasts about 10 minutes. The eclipse of Io on Europa occurs about an hour and a half later, at 10:10 p.m. EDT, also lasting approximately 10 minutes. (All occultations and eclipses described here last around 10 minutes.)
The next two events occur on September 22 and 29. These appearances are better for viewers in the western United States. The September 22 occultation is timed to begin at 8:00 p.m. PDT, and the eclipse begins at 9:39 p.m. PDT. On September 29, Io occults Europa at 10:18 p.m. PDT and eclipses Europa just after midnight at 12:06 a.m. PDT.
The Moon in September 2009
On the evening of September 2, the Moon appears near Jupiter. Two days later, on September 4 at 12:03 p.m. EDT, the full moon occurs. Because the full moon occurs early in September this year, the Harvest Moon is not until October 4, which is closer to the fall equinox. On September 23, the moon is paired closely with Antares and sits within Scorpius's claws. On September 26, the moon appears very close to a star named Nunki, which is part of the handle of the teapot asterism of Sagittarius.
Jupiter's moons and Earth's moon provide the most interesting observing opportunities for September 2009.
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