Go Stargazing! December Edition

Creative Commons License photo credit: chipdatajeffb

Jupiter is the brightest thing in the evening sky this month, except for the moon. Face southwest and look for the brightest point of light there. Mercury makes one of its rare appearances this month. Look southwest at dusk right over the point of sunset to see it. Mars is now high in the west at dawn. It is also brightening as the Earth approaches it. Earth and Mars will align late next month, putting Mars in our sky all night long. Saturn is now high in the east at dawn. Venus is beginning to pass on the far side of the Sun from our perspective. It is now so close to the Sun in our sky that it doesn’t rise until after dawn. Look very low in the southeast in morning twilight to see it.

The enormous Summer Triangle, consisting of the stars Deneb, Vega and Altair, sets in the west. The Great Square of Pegasus is overhead at dusk. The star in its upper left hand corner is also the head of Andromeda. Facing north you’ll see five stars in a distinct ‘M’ like shape—that is Cassiopeia, the Queen. Her stars are about as bright as those in the Big Dipper; she is directly across the North Star from the Dipper. Since the Dipper is low and out of sight at dusk this month, Cassiopeia rides high in the sky.

Dazzling Orion rises in the east, reminding us that winter is on the way. His belt points up to Aldebaran, the eye of Taurus, the Bull. The Dog Stars Sirius and Procyon rise in the early evening, they rise earlier and earlier as the month goes on, and will appear in the sky below Orion in the east. Sirius is the brightest star we ever see at night.

Moon Phases in December 2009

Full moon                 Dec. 2, 1:31 a.m.
Dec. 31, 1:13 p.m.

Last quarter             Dec. 8, 6:15 p.m.

New moon                 Dec. 16, 6:02 a.m.

1st quarter              Dec. 24, 11:35 a.m.

The new moon of Dec. 16 happens to mark the Islamic New Year. 1 Muharram 1431 A.H. actually begins a few days later, when the crescent moon is first seen just after sunset.

At 11:47 a.m. on Monday, Dec. 21, the Earth’s North Pole is tilted away from the sun as much as possible, making the sun as low in our sky as possible. This is the winter solstice, with less daylight for us than any other day of the year. Although December 21 is the shortest day, it does not have the earliest sunset. At Houston’s latitude, that occurred yesterday (December 2).

Creative Commons License photo credit: Shutr

Earth is now about a month away from its closest approach to the sun (perihelion). As it comes slightly closer to the sun, it speeds up a little. This causes sunrise, local noon, and sunset to happen a little later each day. The noon sun won’t get much lower than it is now; we are already that close to the solstice. During December, the effect of Earth speeding up dominates. In the next few weeks, sunsets will occur a little later and sunrises a few minutes later each day. Since most of us sleep through sunrise and see sunset, days will seem to lengthen between now and the solstice, when they’re really getting a little shorter.

If December 21 is sunny, come join us on our museum sundial at noon for solar observing! If not, you can still observe the sun on our sundial near noon any clear day in December or even early January. The holes on our gnomon aligned with the winter solstice are exceptionally large to allow for the cloudier winter weather.