Northern Hemisphere gazer's guide

All month: Our evening sky is beginning to look good this month, not with the splendor of last summer, when Mars, Saturn, and Jupiter were ''playing tag'' with Spica, but still pretty good. Saturn formally joins the cast of evening stars on the 21st, and Jupiter will enter in late May. But both are now up virtually all night, Saturn from dusk till dawn, Jupiter about 21/2 hours behind it (to its east or left). Both are in the east at night, the west at dawn. And now Venus is becoming more noticeable in the west during twilight and later. Three things enhance its visibility this season: its northerly position in the sky, its increasing separation to the left of the sun, and its increasing brilliancy.

Mercury and Mars are also evening objects; Mars is much too close to the sun now. Mercury is probably at its best this year as an evening star, though that is by no means spectacular! Most favorable conditions will be from April 15 to April 23, when the planet's brightness and its elongation (angular distance) from the sun are best. Look in the evening twilight, low in the sky, to the right (north) of where the sun went down.

April 1-2: Moonrise tonight is about 10:30 p.m. The waning gibbous moon, three days past full, comes up in Scorpius shortly before Jupiter. Both moon and planet climb the eastern sky past midnight, the moon more slowly so that the distance between the two closes noticeably. At about 8 a.m. Eastern standard time (EST), the moon slides in front of Jupiter, covering it (an occultation) in the sky over North and Central America and part of South America. In the Eastern United States and Canada the event, likened to a total eclipse of Jupiter by the moon, occurs in daylight; it occurs in twilight in Western areas.

April 4: The morning moon is in Sagittarius, above the Teapot.

April 5: Last-quarter moon is east of the Teapot this morning.

April 6: The moon is at apogee, farthest from Earth, in Capricornus in the morning sky.

April 9: Mercury and Mars are in conjunction, both evening stars poorly placed for viewing, dim and setting soon after the sun.

April 11: You may still be able to see the slender waning crescent moon early this morning, near the southern border of Pisces with Cetus. The vernal equinox is above it and a bit to the west.

April 13: New moon, close to the junction between the constellations Pisces, Cetus and Aries.

April 14: Mercury is in conjunction with the moon only a day or so after new moon, too close to the setting sun to be seen.

April 15-16: You will see the new crescent moon in the west, during twilight as early as the 15th, surely on the 16th, depending on the weather. It is in Taurus, nearly in conjunction with the Pleiades (the closely gathered cluster of stars known as the Seven Sisters) on the night of the 15th, above Aldebaran (the bright-reddish star of Taurus's left eye) on the 16th. Venus is above the moon on the night of the 15th, to its right (west) on the 16th. It should be possible to see the bright planet on the evenings of both dates, low in the west southwest, but it sets only about 21/2 hours after the sun.

April 20: First-quarter moon, in Cancer, occurs this morning.

April 21: The perigee moon (nearest Earth) is near the border between Cancer and Leo. Mercury is at greatest easterly elongation today (greatest distance to the sun's left). In this position it remains above the horizon after sundown, best located for viewing as an evening star. Look for it low in the west during late twilight for about three or four days before and after today. Saturn is at opposition with the sun, above the horizon from sundown till sunrise. It is now an evening star in Virgo to the left (east) of its brightest star, Spica.

April 23-24: Communities that change to daylight-saving time lose that hour of sleep tonight as they advance their clocks one hour.

April 26-27: Saturn rises near sundown on the 26th, just below the nearly full moon, in Virgo, then moves up the eastern sky with the moon. About 2 p.m. EST, the moon passed above the planet and then drifted slowly to its left (east). You can easily see it separating slowly from Saturn on the night of the 26th. Toward morning, the moon becomes full after it moves into Libra.

April 28-29: The moon is in conjunction with Jupiter at about 2 p.m. EST on April 29, the second conjunction of the two this month. And as on the earlier occasion (April 2), the moon occults Jupiter, covering it in the sky over Asia and the western Pacific. On the evening of the 28th, the moon is moving from Libra into Scorpius, to the right (west) of Jupiter. On the 29th, the moon is in Ophiucus, to the left (east) of Jupiter and closer to the planet.

April 30: Just three days after full, the waning gibbous moon enters Sagittarius again, above and to the right of the Teapot.

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