Northern Hemisphere gazer's guide

All month: Jupiter and Saturn are still well-placed evening stars in September, but the planetary scene is shifting rapidly to the morning sky, where Mercury, Venus, and Mars take over toward the end of the month.

Saturn, in Virgo to the left (east) of the bright star Spica, and Jupiter, in Scorpius above Antares, come out of the western twilight soon after sunset, but set early. You will notice their setting times become noticeably earlier during the month as the sun moves toward them at the rate of a half-hour a week. This is Saturn's last month in this cycle as an evening star; Jupiter lasts through October.

Venus entered the morning sky late last month, and Mercury doesn't move to the sun's right until mid-September. But both are exceptionally well placed as morning stars, quickly rising early enough to be seen before sunrise. The same circumstances favor Mars in the morning sky. The steep inclination of the planetary orbits to the eastern horizon at dawn and the very rapid brightening of Mercury and Venus after they pass between sun and Earth make it a very favorable morning elongation for both. Look for Venus and Mars at dawn from midmonth on, for Mercury in the last week of September.

Events below are expressed in local standard time (be sure to correct for daylight) unless otherwise indicated.

Sept. 1: The moon, just one day past last quarter, rises tonight, shortly before midnight, just inside the western border of Gemini. In the morning sky before dawn, the thick crescent moon is midway between the reddish star Aldebaran (in Taurus, to the right) and the ''twin'' stars Pollux and Castor (in Gemini, to the left).

Sept. 4-5: The moon passes Mars (conjunction) at about 9 p.m. Eastern standard time on the 4th, Venus about 11 a.m. EST on the 5th, and it is at perigee (nearest the Earth) at midnight EST (2400 hours) on the 5th. You may still see the crescent moon at dawn on the 5th, but not on the 6th. Mercury and Venus are too near the rising sun to be seen on either morning.

Sept. 8: The new crescent moon might be visible tonight if the southwestern sky is clear. It will be low, but bright, setting in the late twilight. After dark, look for Saturn following the moon down, and then Jupiter, brighter and higher in the southwest, setting about an hour and a half after Saturn.

Sept. 10-11: The moon is between Saturn and Jupiter, closer to Saturn (below and to its right) on the evening of the 10th, to Jupiter (above and to its left) on the 11th.

Sept. 12: Tonight, a day before reaching first quarter, the crescent moon is a spectacular sight with Jupiter in the southwest.

Sept. 14: Venus ought to be visible as a morning star by now, well up in the east at dawn. Look above it and a little to its left early this morning, and you may see Mars as a morning star, but not very bright.

Sept. 15: Mercury is in inferior conjunction, passing between sun and Earth, shifting from left to right of the sun to become a morning star.

Sept. 20-23: Watch the rising moon on these nights. It appears above the horizon very close to the time of sunset, nearly full in phase, filling the eastern evening sky with bright scattered moonlight. The time of moonrise occurs less than half an hour later night to night, about half the average retardation. This effect of brightening the early evening for several successive nights gives the name ''harvest moon'' to the full moon of Sept. 22, occuring at 1:36 a.m. EST. The effect is enhanced the closer the full moon is to the vernal equinox, and this one occurs only hours after passing it.

Sept. 23: The sun is at the autumnal equinox today at 2:42 p.m. EST, marking the end of the summer season and the beginning of autumn in the Northern Hemisphere. As experience shows us, however, this by no means brings an end to hot, summerlike weather.

Sept. 24: Ordinarily we think of five planets bright enough to be seen without a telescope (Mercury, Venus, Mars, Jupiter, and Saturn). There is a sixth, Uranus, but it is barely at the threshold of naked-eye visibility under ideal conditions, seldom realized in today's sky, so obscured by the effects of technology. It is easily visible in even small binoculars if you know where to look, however. At 5 p.m. EST, Jupiter passes above Uranus at a distance less than the moon's diameter. After dark, Jupiter is easy to find in the southwest. With the bright planet centered in the binocular field, Uranus will be a dim starlike object almost exactly below it.

Sept. 26: The sun was at the autumnal equinox last Friday, but this is the day on which day and night are equal and 12 hours elapse from sunrise till sunset.

Sept. 28: The constellation Leo is in the morning sky these days, in the east where the dawn brightens. Its brightest star, Regulus, is well above the eastern horizon. Look near it this morning, and you may see Mars close by above it, about half as bright as Regulus.

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