Astronomy news for the week starting Friday, December 18, 1998.
The week begins the countdown to Christmas with a new Moon that
will wax to first quarter on Saturday the 26th. On Christmas Eve
the Moon will be nicely to the west of Jupiter, on the evening of
Christmas day to the east and below the Great Square of Pegasus.
The Earth and Sun take center stage this week, as the Sun, on its
apparent path along the ecliptic, passes the winter solstice in
Sagittarius at 7:56 PM Central Standard Time the night of Monday,
December 21st. At that moment, astronomical winter will begin in
the northern hemisphere, summer in the southern. At the time of
this solstice, the axis of the Earth will lean as much away from
the direction of the Sun as possible, the Sun will be as low as
possible as seen from the northern hemisphere, 23.44 degrees south
of the celestial equator, and will pass overhead at the tropic of
Capricorn at 23.44 degrees south latitude.
That the southern tropic is named after the constellation
Capricornus even though the Sun is in Sagittarius at the time of
the solstice is one of the results of the precession, or wobbling,
of the Earth's axis, which causes the north celestial pole to move
in a circle of 23.4 degrees radius over a 26,000 year period and
the solstices and equinoxes to move westerly through the stars.
The winter solstice indeed used to be in Capricornus, but some
thousands of years ago moved into Sagittarius. Similarly, the
summer solstice is in Gemini, even though the northern tropic is
named after Cancer, one constellation to the east of Gemini. For
the same reason, Aries tops newspaper horoscope listings even
though the vernal equinox is now in Pisces and will eventually
The early morning sky holds two nice surprises. With the Sun
having cleared Scorpius, bright Antares is visible low in the
southeast just as dawn begins. Better, the planet Mercury is
making an excellent appearance in dawn hours, and on the morning of
Tuesday the 22nd will pass to the north of Antares. The evening
sky also holds a delight, as Venus begins to become visible in
western twilight, the planet to be with us throughout the spring
and much of the summer.