September Sky Chart
Sept. 1: The waxing gibbous moon is in Aquarius, above the horizon at sunset, lighting the night well past midnight.
Sept. 3: Full moon occurs in the early morning, and at night the moon moves below Pegasus. Early in the morning, it is below the western (right-hand) edge of the Square of Pegasus, with the stars Markab and Scheat above it and Fomalhaut (in the Southern Fish) below it.
Sept. 4: Only a bit more than one day past full, the waning gibbous moon rises shortly after dark almost exactly at the Vernal Equinox, in the constellation Pisces. Mark the moon's position relative to Alpheratz and Algenib , the two stars along the eastern (left hand) edge of the Square, and you can easily remember from those stars where the Vernal Equinox is located. The sun is in that position among the stars each year on March 20 or 21, when spring begins in the Northern Hemisphere.
Sept. 6: Mercury is at its greatest distance to the left of the sun today (greatest easterly elongation). This ordinarily puts the planet in its best position as an evening star, but this is a poor elongation. Mercury is too low at sundown and sets too early to be seen, unless you are blessed with an exceptionally clear western horizon during evening twilight.
Sept. 7: Venus may still be seen as a morning star, but very low in the east in the late morning twilight. The star very close below it this morning is Regulus, in Leo. Venus passes from west to east (right to left) above Regulus about 4 a.m. Eastern Standard Time (EST).
Sept. 9: One day before last quarter, the moon is just above Aldebaran when it rises. The reddish star is in the V-shaped cluster called the Hyades, in Taurus.
Sept. 10: Last quarter moon, rising shortly before midnight between El Nath (above) and Zeta Tauri, the two second-magnitude stars that represent the ''horns'' of the Bull.
Sept. 13: The stars above and in line with the crescent moon early this morning are Pollux (the brighter) and Castor in Gemini.
Sept. 14: Look in the dawn sky for Regulus, in Leo, just below the waning crescent moon.
Sept. 17: New moon, in Virgo.
Sept. 19: The young crescent moon should be visible in the east at dusk (weather permitting). Saturn and Spica are below it, both about the same brightness. Spica is the lower of the two. Jupiter, much brighter, is above and to the left of the moon. Mercury begins its retrograde (westerly) motion through the stars today as it heads in between Earth and sun.
Sept. 20: The crescent moon moves to the left above Jupiter at about 2 p.m. EST. After dark tonight, the easily recognized bright planet is below the moon.
Sept. 20-21: To the right and below the moon, past Jupiter, you can see that Saturn moves from right to left past Spica from the evening of the 20th to the 21st. Remember last January when we told you about this year's triple conjunction of Saturn with Spica? That was when Saturn also moved to the left past Spica, but then it swung around in February and moved right on by Spica again, going to the right (west). The last of these three events takes place shortly before midnight on Sept. 20. Now you won't see Saturn pass Spica again for nearly 30 years, the time it takes the planet to move around the sky.
Sept. 21-22: The crescent moon passes Mars about 9 a.m. EST on the 22nd. Watch it shift from right to left above the planet between the early evening of the 21st and 22nd.
Sept. 23: The sun is at the Autumnal Equinox at 3:46 a.m. EST, and autumn begins in the Northern Hemisphere. Since the sun is on the equatorial plane, it is supposed to rise due east and set due west exactly 12 hours later. It does this technically, relative to the celestial coordinate system, the ''imaginary'' lines on the sky used to identify positions. But it doesn't actually because of the circumstances in which we observe these data from Earth.
Sept. 24: First quarter moon is in Sagittarius, above the ''spout'' of the imaginary ''Teapot'' formed by its stars.
Sept. 27: Twelve hours elapse from sunrise to sunset today according to the tables of rising and setting phenomena, because we measure from the upper edge of the sun and take refraction (by the atmosphere) into account.
Sept. 30: Tonight's waning gibbous moon is up at sundown, again midway between the Square of Pegasus and Fomalhaut, where it was on Sept. 3. But note that, having gone completely around the earth in the past 27 days, it isn't full as it was last time it was here. Full moon doesn't occur for another three days. Can you see why?
All Month: Not a very good month for planets, but Jupiter is beautifully placed as an evening star, coming out of the evening twilight shortly after sunset, bright and easy to see until it gets too low shortly after dark. Mars isn't nearly so bright, but it becomes visible after dark well to the left of Jupiter. Watch it move swiftly through the stars from night to night toward Antares in Scorpius, nearly reaching it by the end of the month