Photo of the Week. A waning gibbous Moon near its
third quarter glides down the daylight western sky.
Astronomy news for the week starting Friday, December 7, 2007.
We begin our week with the Moon as a waning
crescent just shy of new, that
transition phase to waxing crescent
taking place on Sunday, December 9, around noon in North America.
The waxing crescent will be too close to the Sun and too thin to
see that night, but with luck and a clear sky you might find it the
following night in twilight in the southwest just after sunset. By
the evening of Monday the 10th, it will be obvious as it grows
toward first quarter, that phase not
reached until Monday the 17th. The evening sky is now dominated by
Mars, which rises in Gemini in
twilight just before 6 PM. Transiting the meridian high to the south around 1:30
AM, the red planet will have moved well into western skies by the
time Venus rises two hours later. Venus then owns the southeast
until growing dawn finally takes over. In between, Saturn rises in
southern Leo about 11 PM.
And with binoculars or a telescope you can still watch Comet Holmes as it loops through Perseus.
Three other items occupy the week.
Uranus's rotation axis is tilted by 98 degrees relative to its
orbital path (the Earth's tilt a mere 23.4 degrees). As a result,
Uranus has the
most extreme seasons of any planet in the Solar System: the Sun can be nearly
overhead as seen from the rotation poles. Over the past two
decades, as the Sun has crept toward the equator (as seen from the
planet), weather patterns have strengthened. On Friday the 7th,
the Sun will cross the Uranian equinox, the Sun finally showing up
overhead at the Uranian equator as it enters the opposite
hemisphere.
Next, while the shortest day of the year takes place at the time of
the Winter Solstice passage, this
year on the night of December 21, December 7 is the usual date for
the earliest sunset. The difference is caused by the eccentricity
of the Earth's orbit (which causes the Earth to move in orbit at a
variable rate and the Sun to move similarly at a variable angular
rate on the ecliptic)
combined with the axial tilt. Latest sunrise then occurs about two
weeks after the solstice.
Finally, you might wish to watch one of the best meteor showers of
the year, the Geminids.
Seeming to come out of Gemini, they increase in strength during the
week, and will peak the morning of Friday the 14th with a maximum
rate in a dark sky of up to 120 per hour. The Moon cooperates by
being well out of the sky. The Geminids are the product of Phaeton
, a defunct comet that was once thought to be an
asteroid.
This is also a wonderful season for admiring Cassiopeia, the Queen, whose characteristic "W" rides
high across the meridian a bit to the north of overhead around 8
PM. Look to the east of it to find the faint smudge that is the Double Cluster in Perseus, then
go on to seek Comet Holmes.
