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All month: Mercury, Jupiter, and Saturn are evening stars in August; Mars is a morning star; and Venus swings from the evening to the morning sky when it goes through inferior conjunction on Aug. 25. But only Jupiter and Saturn can be seen this month. The other three ''naked eye'' planets are not well placed for viewing.

Jupiter and Saturn are exceptionally prominent in the western sky from after sundown till they set in the early evening. Saturn is low in the southwest at dusk, very similar in appearance to the star Spica, in Virgo to Saturn's right and below. Jupiter is higher and brighter, the brightest starlike object in this summer's sky. It becomes visible in early twilight near the south, following Saturn down by about two hours.

Events below are expressed in local standard time unless otherwise indicated.

Aug. 1: Last-quarter moon occurs today, at the southern border of Aries. But moonrise isn't until close to midnight, standard time. In compensation, of course, the moon is a nice daytime object during morning daylight hours. Venus accelerates the shrinking of the distance between it and the sun as it begins its westerly (retrograde) motion. This hastens also its rate of disappearance from the evening sky. Bright as it still is, you won't see it around more than another week or so.

Aug. 2-3: The morning moon is in Taurus these days, below the stars of the Pleiades before dawn on the 2nd, a trim crescent above the reddish star Aldebaran in the V-shaped Hyades cluster on the 3rd.

Aug. 6: The fast-waning crescent moon is now in Gemini, and if you look for it early this morning (it rises about 3 hours before the sun) you may see the ''twin'' stars, Pollux and Castor, coming up about 45 minutes later. A dim reddish object below Pollux (by about the same distance as that between the twins) is Mars, and this could be the first chance for you to see it as a morning star this cycle, if the eastern sky is exceptionally clear.

Aug. 8: The moon is at perigee (nearest Earth) at about 2 p.m. Eastern standard time (EST) within minutes of its becoming new moon. When perigee occurs close to the times when sun, moon, and Earth are in line (as with new moon), the effect of perigee can advance or retard the times of high and low tides and sharply increase the range of the tide from low to high. The resulting ''higher'' high tides can cause severe flooding in low-lying coastal areas if conditions are appropriate. Since perigee and spring tide virtually coincide today, we can expect much higher tides during the night and tomorrow morning.

Aug. 11-14: The nearly new moon is a blessing for meteor watchers looking for the Perseids shower in the after-midnight sky on these dates. The crescent moon sets each night before midnight, leaving the morning sky dark. Best dates should be the 12th and 13th, when you might expect to see 40 to 50 meteors an hour, many quite bright. But you can still expect 25 or so an hour on the 11th and the 14th. Remember, meteor watching is best after 1 a.m. Always choose a dark area with as much of the sky as possible in view; and allow your gaze to drift around as much of the sky as possible. The meteors can appear anywhere, even though they seem to radiate from the area in Perseus from which they get their name.

Aug. 12: From the moon in the southwest, look up the sky to the left to Spica (the lower) and Saturn, then further left to Jupiter (brightest of all) and Antares in the south.

Aug. 13: Saturn (nearest) and Spica are in line below the moon.

Aug. 15: After passing first quarter at 7:47 a.m., the moon rises very close to Jupiter, passing it about 1 a.m. EST on the 16th.

Aug. 18: Tonight's gibbous moon is just above the peak of the Teapot's lid. Look below the moon for these stars marking the constellation Sagittarius.

Aug. 22: The moon is at apogee, farthest from Earth.

Aug. 24: At midnight EST, Venus ends this cycle as an evening star, passing between sun and Earth (inferior conjunction) and entering the morning sky.

Aug. 25: When it rises tonight (about 8:30 p.m.), the gibbous moon is just south of the vernal equinox. Look above the moon also for the Square of Pegasus , four equally bright stars arranged in a large box.

Aug. 28: Look at the moon tonight after it rises about 9:15 p.m. On the southern border of Aries, it is just about where it was on Aug. 1, having gone once around the sky as it moves around Earth. But on Aug. 1, it was at last-quarter phase and tonight it is obviously fatter - gibbous in phase, about three days before last quarter. Can you explain why?

Aug. 31: The moon is at last-quarter phase today, and tonight it is in Taurus , when it rises about 11 p.m. Editor's note: The sky map in the July issue shows the evening constellations and stars for this month and gives the times for use.

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