December 2004 Skies

Several remarkable solar system and stellar sights are visible in our December s skies.  Of much interest are two planetary conjunctions visible in the morning sky and the Geminid Meteor Shower, typically Arizona’s finest, which is visible in a dark (moonless) sky.

December sees our Sun reach its lowest noontime height above the southern horizon for the entire year.  By night the prominent Winter stars and constellations make their entrance for convenient viewing.

The SUN begins the month in the constellation of OPHIUCHUS, and crosses it until December 18th, when it enters SAGITTARIUS, where it remains for the rest of the month.

The Evening Sky:  Early in December sunsets occur at about 5:15p.m. (our earliest of the year).  Because Earth’s orbit around the Sun is not a circle, but slightly elliptical, in December Earth is closest to the Sun and our planet’s orbit speed is highest.  As a result, the Sun’s setting time seems to stand still for about two weeks and change only slightly for the following two weeks. At month end, sunset will occur only 10 minutes later (at 5:25p.m.).

The MOON begins December five days past Full and remains in the morning sky for the first third of the month.  New MOON occurs shortly after sunset on Saturday the 11th in the constellation of OPHIUCHUS.  With a clear view down to the Southwestern horizon on Monday the 13th, you may see a very thin crescent Moon in SAGITTARIUS beginning at about 5:45pm.

On the evening of the 23rd, the Moon will be located in the Eastern sky in TAURUS near the PLEIADES (“Seven Sisters”) star cluster.

Technically, Full Moon occurs near sunrise on Sunday morning the 26th in the constellation of GEMINI, but on that evening Moonrise is only 10 minutes after sunset. The next evening (27th), the Moon rises near planet SATURN.

Evening Planets:  MERCURY and SATURN are December’s Evening Planets. 

Mercury begins the month about 10 degrees to the East of the Sun in Ophiuchus,  However, Mercury quickly moves Westward and catches the Sun on December 10th, when the planet moves into the Morning Sky.

Saturn continues to rise earlier each evening, beginning the month by rising at 8:00pm. and ending 2004 by rising at 6:30 pm, only about an hour after sunset.  Saturn spends the entire month in RETROGRADE MOTION in Gemini, moving slowly westward with respect to the background stars, and slowly approaches its closest distance to the Earth in early January.

The Morning Sky:  Arizona sunrises occur at about 7:15a.m. on December 1st, and as late as 7:35a.m. on the 31st.  Our early sunsets and late sunrises combine to make December’s days the shortest of the year.

The waning MOON is visible in the morning sky through the 10th of the month, as it passes close to several bright stars and planets .  On the 7th, the Moon will rise covering Planet JUPITER (which emerges shortly after moonrise).  On the mornings of the 9th and 10th, the Moon will be near both the planets Venus and Mars.

Morning Planets.  MERCURY, VENUS, MARS AND JUPITER are December’s Morning Planets.  Two impressive morning sky planetary conjunctions will occur in December (see below).

JUPITER begins the month high in the Southeast at dawn, rising at 2:45a.m.  Jupiter will remains in VIRGO close to the bright star Spica all month.  Again, look for Jupiter very close to the Moon on the morning of the 7th.

VENUS rises a minute or two before 5:00a.m. on December 1st in the constellation LIBRA, and rises rises an hour later (about 6:00 a.m.) at month end.  During December Venus moves Eastward each day toward the Sun, crossing Scorpio and moving into Ophiuchus to meet the Sun early in 2005.

MARS rises at 5:10a.m. on the 1st and is easy to see as a bright reddish object near brilliant white Venus.  Mars catches up with Venus in early December and the two bright and contrasting colored planets form a brilliant pairing early in the month.  Watch Mars pass Venus low in the Southeast between December 3rd to 7th.  The planets stand nearest in the sky (about 1 degree of arc apart) on the morning of the 5th.

MERCURY should reappear in the morning sky about December 20th.  For a month thereafter, watch for Mercury and Venus to dance around one another in the Southeastern dawn sky.  Mercury begins to pass Venus on the 27th enroute to looping around it in the sky for a very close conjunction in early January.

What Else to See?  In addition to the close Moon-Planet conjunctions noted above, December is most noted for convenient viewing of winter constellations, the winter Milky Way, and the Geminid Meteor Shower.

December marks the first month for highly convenient viewing (before the evenings become too cold) of the Winter Milky Way constellations CASSIOPEIA, PERSEUS (with the famous “Double Cluster”), AURIGA and GEMINI.  If viewing from dark sky locations, many star clusters are visible along the Milky Way to the unaided eye. Orion, with its famous “Great Nebula” (AKA Messier 42) rises shortly before sunset on December 1st and rises higher each evening in an impressive wintertime display. Follow the line of Orion’s 3 prominent Belt stars upward to the red star Aldebaran in Taurus.  Then look below Orion’s Belt to Sirius (the brightest star in our sky) in the constellation of Canis Major. Directly to the left of Orion will be Gemini, with bright stars Castor and Pollux and planet Saturn.  Below and to the left of Orion is Canis Minor and the bright star Procyon.  Both Sirius and Procyon are double stars.  Although we see only the brighter star in each pair, there is actually another faint, highly evolved “White Dwarf” companion star orbiting Sirius and Procyon. Because Sirius and Procyon are so bright, it requires a fairly large telescope to see their companions.

The Geminid Meteor Shower is an annual event, but is spoiled when the Moon is high and bright.  Since the Moon will be near New this month, it will not interfere with the show!  For about four days, on the nights of December 10th through December 13th  numbers of meteors will be seen to radiate from the zodiac constellation Gemini.  At the shower’s peak on the evening of the 12th, perhaps 60 meteors per hour (1 per minute) can be seen by an observer in a dark location.  These meteors are remnants of a long-gone comet whose orbital debris dust intersects the Earth’s orbit during these December days.  Shower meteors never reach the Earth’s surface, but the Geminids do provide many bright meteor trails, some with bright green or yellow trails and an occasional persistent vapor cloud.  Meteor shower observers should first download a star chart/sky map (see below) and then stay comfortable and warm while observing.  A pool lounge chair with adjustable back/head angle would be ideal.  Face a direction with no visible streetlights, etc.  for best results. Even though the meteors radiate from Gemini, some may be seen in every direction in the sky.

On Friday, December 10th beginning at 6:00p.m., the East Valley Astronomy Club is hosting a Free Public Star Party at the Riparian Preserve next to the Gilbert Public Library.  Several telescopes will be available for public viewing of interesting wintertime astronomical objects.  The Club will also sponsor an astronomy lecture at 8:00 p.m. inside the Library.  Admission to the lecture is $3.

Good Luck Observing!