The Sky in September
Rao, Joe, Natural History
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 skywatchers 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. …