The sky in September

Natural History, Sept, 2004 by Joe Rao

Mercury makes a brief appearance in the September sky, peeking out from the glare of the Sun in the first week of the month. On the 9th, shining at magnitude -0.4, the swiftest planet reaches its greatest elongation, eighteen degrees west of the Sun, and rises with the break of day. The following morning Mercury is up ninety minutes before the Sun and passes breathtakingly close to the star Regulus. Observed from Europe, the planet almost grazes Regulus, passing just 0.06 degree (about an eighth the diameter of the full Moon) south of the star. For most viewers in the Western Hemisphere, Mercury has already begun to recede from the star by the time the planet rises, though it is still less than half the Moon's disk away. Speedy Mercury is easily visible as late as the 19th; as it nears the Sun on the sky, it brightens to magnitude -1.2. Thereafter, the planet rises invisibly in the glare of the morning.

Venus rises about 3 A.M., some two hours before the first light of dawn, and shines near the much dimmer Saturn as the month begins. For sky watchers at midnorthern latitudes, morning apparitions of Venus, shining at magnitude -4.2, don't get much better than this. The planet glides about three and a half degrees south of the center of M44, the Beehive star cluster in the constellation Cancer, the crab, on the mornings of the 10th and 11th. By month's end Venus has descended to within five degrees of Regulus, on its way to a close encounter on October 3.

Mars, is in conjunction with the Sun on the 15th and cannot readily be seen until the end of October.

Jupiter, too, is lost in the glare of the Sun during September and reaches conjunction with our star on the 21st.

Saturn starts the month paired with blazing Venus. On the 1st, look toward the east-northeast soon after 3 A.M.; Saturn is a couple of degrees above and to the left of Venus. Even farther above and to the left of this planetary pair are Castor and Pollux, the bright twin stars of the constellation Gemini. Saturn shines at magnitude 0.2 but pales next to the brilliance of Venus. Indeed, Saturn only appears one-fifty-eighth as bright. Venus gradually moves east as September ages, leaving Saturn on its own in Gemini. The planet's ring system is tipped about twenty-two degrees toward Earth, and provides a spectacular view even in small telescopes.

The Moon wanes to last quarter on the 6th at 11:11 A.M. and becomes new on the 14th at 10:29 A.M. It waxes to first quarter on the 21st at 11:54 A.M. and to full on the 28th at 9:09 A.M. That full Moon is known as the harvest Moon: the full Moon closest to the autumnal equinox in the Northern Hemisphere.

The equinox takes place at 12:30 P.M. on the 22nd. The Sun crosses the celestial equator from north to south as it traces its apparent annual path against the background of stars. Autumn begins in the Northern Hemisphere, spring in the Southern.

All exact times are given in eastern daylight time.