Magazine article Natural History

The Sky in September

Magazine article Natural History

The Sky in September

Article excerpt

Mercury remains a poor evening apparition in the Northern Hemisphere all this month, despite the increase in its angular separation east of the Sun throughout most of September. That's because the planet sets unfavorably early, only about forty-five minutes after sunset. Nevertheless, Mercury is worth a look with binoculars on the evenings of the 21st and 22nd. That's when it makes a close approach to Spica, the brightest star in the constellation Virgo, the virgin. Look for the pair just a few degrees above the west-southwestern horizon about thirty minutes after sunset.

Venus erupts into view in the eastern morning sky as September opens, rising just after dawn's first glow at around 5:00 A.M. local daylight time. With each passing day, this morning "star" rises higher and becomes a little brighter. By month's end, it is rising at around 3:30 A.M., some three and a half hours before sunrise. Venus was at inferior conjunction on August 18, in line between the Earth and the Sun. Now it is swinging away from that line, speeding ahead of Earth in its faster orbit. So through a telescope, the planet displays a large, brilliant crescent that wanes in phase all month while it shrinks in size.

Mars rises about four and a half hours after sunset and is high in the south-southeast by dawn. The planet begins the month in the constellation Taurus, the bull, seven and a half degrees to the lower left of the bright star Aldebaran-easy to mistake for Mars because of its similar hue. By month's end, Mars has shifted eastward to the feet of the constellation Gemini, the twins. We are slowly catching up to Mars in our orbit around the Sun. As a result, the Red Planet brightens by almost 50 percent this month, from magnitude +0.3 to -0.1, and, viewed in a telescope, its apparent size is slowly growing.

Jupiter, shining steadily in the southwest, five or six degrees above the ruddy but much dimmer star Antares, is the first "star" to appear in the darkening sky. …

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