September opens with Venus hosting winter constellations in the eastern predawn sky.

September opens with Venus hosting winter constellations in the eastern predawn sky. West of the planet, the bright star Procyon, in Canis Minor, the little dog, rises at almost the same time as Venus, followed about 40 minutes later by the brightest of stars: Sirius, in Canis Major, the big dog.

As the month goes on, the winter stars sweep westward; Venus drops toward the sun, rising later every morning; and the spring constellation Leo, the lion, becomes the planet's new starry companion. In mid-September, Mars and Mercury enter the morning sky over the eastern horizon. Mars is distant and dim, and it climbs slowly. But nearer, brighter Mercury, as usual, quickly pops (up) and drops. On the 16th, Mercury passes close to Mars in the sun's foreglow.

But better is to come. On the 18th, look an hour before dawn to see, stacked from top to bottom: Venus; Regulus, the brightest star in Leo; an old crescent moon; Mars; and Mercury. Mars and Venus are rapidly approaching each other en route to a close pairing in early October.

In the evening sky, Jupiter all but disappears into the sun's afterglow this month. This leaves Saturn, in the south to southwest, the lone evening planet visible to the naked eye. On the 15th, we say goodbye to a longtime companion of Saturn when the Cassini spacecraft plunges into the planet. For nearly 20 years, Cassini has sent us breathtaking images of the planet, its rings and its moons. As it makes its fatal dive, it will relay data about Saturn's atmosphere until contact is lost.

Autumn's "water" constellations are now moving into prime evening position in the south. Capricornus, the sea goat, is westernmost. Moving eastward, we see spidery Aquarius, the water bearer; then the Circlet of Pisces, the fishes. Above the Circlet is the Great Square of Pegasus. Below Aquarius, the bright star Fomalhaut marks the mouth of Piscis Austrinus, the southern fish. Meanwhile, high in the southwest, the Summer Triangle of bright stars encloses a patch of sky replete with constellations big and small, plenty to delight anyone with a star chart.

September's full moon arrived at 2:03 a.m. on the 6th. And at 3:02 p.m. on the 22nd, the autumnal equinox ushers in fall. At that moment the sun crosses the equator, lighting the Earth from pole to pole. Also, from that moment until the spring equinox, the farther north you go, the shorter the day length. The day length is shrinking fastest now, when we’re near the fall equinox, because this is when the sun drops most rapidly southward.

The University of Minnesota offers public viewings of the night sky at its Duluth and Twin Cities campuses. For more information and viewing schedules, see: Duluth, Marshall W. Alworth Planetarium: www.d.umn.edu/planet; Twin Cities, Minnesota Institute for Astrophysics: www.astro.umn.edu/outreach /pubnight; Check out the astronomy programs at the University of Minnesota's Bell Museum ExploraDome: http://www.bellmuseum.umn. edu/exploradome; Find U of M astronomers and links to the world of astronomy at http://www. astro.umn.edu