Astronomy news for the week starting Friday, December 17, 2004.
The Moon, in its perpetual cycle around the Earth, passes through
first quarter on Saturday, December 18, shortly before its
daylight Moonrise in North America, at which time we see half the
sunlit face of the Moon, while the other side -- that facing away
from the Sun -- is in darkness.
The morning sky remains a marvel, with all five ancient planets on
display in order of their distance from the Sun. Near the horizon,
Mercury glows in bright twilight, below and to
the left of brilliant Venus (which now rises in the southwest just
before twilight begins). Up and to the right of Venus, you can
spot reddish Mars in Libra.
Higher still is obvious Jupiter, in Virgo, which now rises around 1:30 AM. Quite a bit
farther over in the western sky is the most distant of the planets
known since ancient times, Saturn, in Gemini. Rising just after twilight ends, Saturn is the
lone bright planet in the evening sky, though the progression
continues with faint Uranus and Neptune (which now set in mid-evening).
Wrapping around to the other side of the Sun (and into the morning
hours again), the progression ends with currently invisible Pluto.
All that is left out is the Earth, which will actually take center stage when
the Sun passes through the Winter
Solstice in Sagittarius at 6:42
AM Central Standard Time (7:42 EST, 5:42 MST, 4:42 PST, 3:42 Alaska, 2:42 Hawaii) on Tuesday, December
21st, about or before the time of sunrise. At that moment, the
Earth's axis is tilted directly away from the Sun, astronomical
winter begins in the northern hemisphere, the Sun is overhead at
the Tropic of Capricorn, is
circumpolar (not setting) at the Antarctic Circle, and barely makes
it to the horizon at the Arctic Circle. Though the Sun will now
begin its long, six-month, trek to the north, the days in the mid-
northern hemisphere will continue to chill as the northerly
movement is at first so very slow such that the solar heating rate
remains low until spring approaches.
The stars of winter join in and are fully with us, Orion now up above the eastern
horizon as evening twilight comes to an end. Up and to the right
of him is an icon of the passage from autumn to winter, Taurus the Bull, whose head is
formed from the Hyades star
cluster and the first magnitude star Aldebaran (which is not actually
a part of the cluster, but a foreground star). Taurus is perhaps
best known for containing the exquisite, more distant cluster
called the Seven Sisters, or Pleiades, of which six stars are readily visible.
Directly above the Pleiades, to the north, is bright Perseus, whose most famous star is the
eclipsing double Algol, the "Demon