Northern Hemisphere gazer's guide

Sept. 1-2: In the morning sky of both days, the last quarter moon is close to Aldebaran, the bright star of Taurus. At dawn on the 1st, the moon is above and to the right of Aldebaran. On the 2nd it is below and to the left of the star. At about 1 p.m. Eastern standard time the moon covers the star (an occultation) over western North America, eastern Asia, and the Pacific.

Sept. 5: A trully spectacular conjunction of Venus and the moon will occur this morning. The waning crescent moon and the planet will be well above the eastern horizon by 3 a.m. Moving east rapidly relative to bright Venus, the moon will overtake and pass it (from right to left) at about 5 a.m. EST. You should easily be able to see the moon slide past Venus in the few hours they are visible. During that time, although their positions will be changing slowly, Aldebaran will seem to stand beneath the crescent's "nether" tip. But it won't, of course, appear within the crescent, a classic artistic blunder, even in the parts of Asia and Europe where the moon moves directly toward the planet. In doing so, it will cover Venus instead, the kind of event called an occultation.

Sept. 7: The moon passes close to the bright star Regulus, in Leo, today, close enough to cover (occult) the star in the sky over Asia and the Pacific. But we won't see them in our sky; the two are very near the sun and rise too late to be visible in the dawn sky.

Sept. 9: We will miss the conjunction of Mercury with Saturn because both are "poor" evening stars, setting before the western sky darkens enough to allow us to see them.

Sept. 12: The moon is at apogee, the position in its orbit where it is most distant from the earth.

Sept. 13: Jupiter is in conjunction with the sun, which passes the planet from right to left, taking Jupiter out of the evening sky to become a morning star.

Sept. 13: The moon passes Mars today at about 1 p.m. EST. At dusk this evening the planet will be almost directly below the crescent moon, but not very bright.

Sept. 22: The sun arrives at the autumnal equinox at 4:09 p.m. EST and autumn begins in the Northern Hemisphere. The sun is located on the equatorial plane today, crossing it from north to south at a point in the constellation Virgo.

Sept. 22: Saturn is in conjunction with the sun and now becomes a morning star.

Sept. 24: The full moon of September this year is the Harvest Moon. For several nights in a row (at least the 23rd through the 25th) the moon will appear to be full or nearly full and will rise very nearly at the time of sunset , ensuring harvesters (and others) bright moonlight in the early evening after twilight fades. The phenomenon occurs always with the full moon closest to the time of the autumnnal equinox (in March in the Southern Hemisphere). Because of the orientation of the moon's orbit to the eastern horizon at sundown, the retardation (delay) in the time of moonrise from one day to the next is substantially less than average.

Sept. 24: Mercury passes very close to Spica, the bright star of Virgo, today , but the two set too early in the evening, before the sky darkens enough to see them.

Sept. 24: Perigee moon (nearest earth) occurs just 15 hours after new moon. At perigee, the lunar tide is enhanced because the moon is closer to earth. The tide at new moon is also stronger than usual each month because the lunar and solar tides reinforce one another when sun, moon, and earth are in line (the socalled spring tides occur twice monthly, at new moon and full moon). When perigee occurs near the time of new (or full) moon, the effect of the perigee tide is added to the spring tide. This happens today, and we can expect exceptionally high tides this evening and tomorrow morning.

Sept. 28: The moon rises near Aldebaran late tonight and remains close to the star (but drifting slowly east) through the rest of the night. At about 8 p.m., before Aldebaran rises in North America, the star is covered by the moon, its second occultation this month. Note that the first occultation (on Sept. 1) occurred when the moon was at last quarter, but today's is two days before last quarter. The difference reflects the time it takes the moon to go once around the Earth relative to the position of a star vs. the time it takes to go through the cycle of phases (relative to the sun). Which do you think is a better measure of the moon's period of revolution around the Earth?

All Month: Venus, the bright evening star of last winter and spring, is now at its very best in the morning sky.