Astronomy news for the week starting Friday, April 6, 2001.

The Moon passes through its full phase this week the night of Saturday, April 7, somewhat after Moonrise in North America. The Moon will therefore rise a bit before sunset, but since the full phase is passed while the Moon is visible, it will also set a bit after sunrise. The night of Thursday the 12th, the waning gibbous Moon will pass to the north of Mars and make a handy reference with which to find the red planet.

Mars now officially moves into the evening sky, rising just before midnight (1 AM daylight time). It is also very close to bottoming out in its current journey along the ecliptic path, having recently entered Sagittarius. Nearly 23 degrees south of the celestial equator, the planet rises well into the southwest. During most of this month, as Mars crosses -- transits -- the celestial meridian to the south, Venus rises in the northwest, but after the beginning of twilight, so the planet will appear low in the sky. On Friday the 6th, Venus passes 10 degrees north of Mercury, the little planet so close to the Sun that it is not possible to see from North America.

The evening sky still provides the best planetary view, with Jupiter and Saturn glorious to the west among the stars of Taurus. Saturn now sets around 10:30 (Daylight Time), Jupiter an hour later, the two slowly drawing apart. Slowly being overtaken by the Sun, the pair will, one at a time, disappear into twilight during the month of May.

Daylight time has just been launched over most of the US and Canada. The time of day refers to the "hour angle" of the Sun (with the effects of orbital eccentricity and axial tilt smoothed out), the angle the Sun makes with the celestial meridian, each hour past noon corresponding to another 15 degrees. Time is really longitude dependent. Standard time is local time at specified meridians. Technically, everyone in a band 15 degrees wide centered on that meridian keeps the same time (though the bands are wildly shifted for political and social reasons). If you are west of your meridian, the Sun seems to set late, while if east it will seem to set early. In daylight time, we shift to the time zone to the east of us, and as a result, the Sun seems to set an hour later than "normal."

Dim Cancer, flanked by Gemini to the west and Leo to the east, crosses the meridian around 9 PM. Above it is even dimmer Lynx, below it the circlet that makes the head of Hydra, the Water Serpent. Much farther down is Pyxis, the Compass, all a challenge and nearly impossible to see under the glare of full Moonlight.
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