Northern Hemisphere gazer's guide
New York — Aug. 31-Sept. 1: On the last night of August and the first day of September the moon passes Saturn, Jupiter, and Venus within a period of 14 hours. On both evenings the three planets will be stretched out in line above the western horizon during evening twilight, Venus the brightest, Jupiter to its right, and then Saturn to the right of Jupiter when the sky is dark enough. All three will set about the end of twilight, within two hours of sunset.
The crescent moon appears among them on the night of Aug. 31, closest to Saturn about 8 p.m. (Eastern standard time) Sept. 1, above Venus nine hours later. When they appear again on the night of Sept. 1, the moon will be to the left (east) of Venus, moving toward Spica, the bright star in Virgo to the east and higher.
Sept. 2: The star to the moon's right this evening is Spica.
Sept. 5: Look below the crescent moon tonight for the reddish star Antares, marking the "heart" of Scorpius. The moon is at apogee today, its greatest distance from the earth.
Sept. 6: Venus has been separating continually from Jupiter since passing it on Aug. 27, moving toward Spica, in Virgo. Today Venus passes above Spica, and will shift to its left (east) on successive evenings. Look for Venus about half an hour after sundown, in the west; look for Spica about 20 minutes later, since the sky must darken more before it can be seen. Jupiter and Saturn are now well below Venus, setting 45 minutes earlier.
Sept. 7: The gibbous moon (one day past first quarter) passes through the constellation Sagittarius tonight. Look below the moon for the "teacup" arrangement of eight stars that identifies the constellation.
Sept. 10-13: Mercury is in conjunction with Saturn on the 10th, with Jupiter on the 13th. Unfortunately, it passes about four degrees south of each planet in turn, and sets less than an hour after sunset. It will be difficult enough to find Jupiter and Saturn now.
Sept. 13: Tonight's full moon is called the Harvest Moon. It seems to illuminate the early nighttime for several nights in a row, making it particularly useful to farmers staying out late to finish the late summer harvest.
Why is this so? It has to do with the so-called "retardation" in the time of moonrise from night-to-night. On average, the moon rises about 50 minutes later each night (because it is moving easterly in its orbit around earth, opposite to the westerly motion across the sky that earth's rotation gives it). But this "lag" can vary from about 30 minutes to about 70 minutes in the mid-latitudes, depending on the angle with the horizon of the moon's orbital path at the time it is rising. This angle, and the resulting retardation, is least when the moon is at or near the vernal equinox, and the full moon is in that position in September, near the time of the late summer and early autumn harvest. The effect is more noticeable at high latitudes; above the Arctic Circle, for that matter, the Harvest Moon actually rises earlierm from one day to the next! When do you suppose the Harvest Moon occurs in the Southern Hemisphere?
Sept. 14: The moon is very near the vernal equinox tonight (it's closest about 10 p.m. EST). Its position can help you mark where the vernal equinox is among the stars.
Sept. 16: The moon is at perigee, the position in its orbit where it is nearest earth.
Sept. 19: The waning gibbous moon rises after 10 p.m. in Taurus, about midway between the "Seven Sistes" (the closely gathered star cluster called the Pleiades) and Aldebaran, the reddish star in the V-shaped cluster called the Hyades (marking the Bull's face).
Sept. 22: The sun arrives at the autumnal equinox, a point in the constellation Virgo, at 10:05 p.m. EST, and autumn begins officially in the Northern Hemisphere (spring in the Southern Hemisphere). Do you think that autumn begins on Sept. 22 everywhere in the Northern Hemisphere?
Sept. 23: Mercury is at greatest easterly elongation, its greatest distance to the left (east) of the sun. Ordinarily this places the planet inthe best position to be seen as an evening star. But this is an exceptionally poor evening elongation. The planet is very low at sunset and sets less than an hour later.
Sept. 23-24: The moon rises at about 2 a.m. EST on the morning of the 24th, very close to the planet Mars. Look for Mars just below the tip of the crescent. Earlier (at about midnight), the moon covers Mars (an occultation) in the sky over portions of Africa, southern Asia, and Australia.
Sept. 26: Although the autumnal equinox occurred on the 22nd, this is the day on which the sun spends 12 hours above the horizon and 12 hours below. On the 22nd, the centerm of the sun was exactly over the equator, but we define sunrise and sunset by the upper edge of the sun, and we allow for the refraction (or bending) of sunlight as it passes through atmospheric layers.
Sept. 29: The early crescent moon is in conjunction with Mercury, but the planet is well south of the moon. Even if you can see the moon tonight (about two days old), forget about looking for Mercury!