SKYLIGHTS
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.
