Astronomy news for the week starting Friday, November 26, 1999.

The Moon, at the start of the week in its waning gibbous phase, passes through third quarter on Monday the 29th rather well before moonrise in North America. Though the third and first quarter Moons have the same reflecting areas, the third quarter is notably dimmer because it is covered by a larger expanse of dark volcanic rock. The lunar lava beds do not come from volcanoes but from seepage through surface rock that was broken by the ancient giant collisions that made the dark impact basins we call the "maria" and that make the popular figure of the "man in the Moon."

The week holds a variety of planetary lineups. Mars, deep in the southwestern evening sky and moving against the stars of Capricornus, passes about two degrees south of much more distant Neptune on Monday the 28th. Completely invisible is the conjunction between Pluto and the Sun on Thursday, December 2, the planet 40 times the more distant of the two. Much more visible is a conjunction between Venus (which dominates the early morning sky) and the star Spica in Virgo, Venus passing four degrees north of the star on the morning of Monday the 29th. At the same time, try to find little Mercury, which passes its greatest western elongation with the Sun on Thursday, December 2. Not easy to see in growing eastern twilight, the planet will below and left of its brilliant cousin Venus and near the horizon. Since the Moon will also be passing Venus during midday on Friday, December 3, it will make a nice pairing with the planet that same morning.

The autumn constellations are now in full evening display. Look high to the south between 7 and 8 PM to find the imposing Great Square of Pegasus. Immediately below it, and just above the celestial equator, is a delightful asterism, the "Circlet" of Pisces, which represents the head of the western of the two mythological fishes. Obvious in a dark sky, the ring of stars is just up and to the right of the vernal equinox, the point where the Sun will cross the equator next March 20 to mark the beginning of northern spring. The vernal equinox, as well as the autumnal equinox and the solstices, are moving slowly to the west against the background of the stars as a result of a 26,000-year "precessional" wobble of the Earth's axis. For the past two millennia the vernal equinox has been creeping through Pisces, having entered from Aries around 100 BC. Seven hundred years from now it will finally exit for Aquarius. While the motion is slow, it can be detected with the naked eye over a lifetime, and has been known since around 150 BC.
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