Saturn is lost in the Sun’s glare this month.
Mercury briefly appears in the evening sky in the last two weeks of December and in early January. Look low in the west-southwest during twilight, just over the point of sunset, to find it.
Venus, Mars, and Jupiter are spreading apart in the morning sky this month. Right now, Venus is in the southeast at dawn, outshining everything but the sun and the moon. Jupiter, not quite as bright as Venus but brighter than everything else, is high in south at dawn by New Year’s Eve. Mars is between Venus and Jupiter and much, much dimmer than those two.
The Summer Triangle sets in the west. Watch for the Great Square of Pegasus almost overhead at dusk now and in the west by Christmas. Taurus, the Bull, rises in the east. Look for the Pleiades star cluster above reddish Aldebaran. Dazzling Orion, the Hunter, rises shortly after dusk (by month’s end, it is already up at dusk). As Orion enters the evening sky, we transition from the relatively dim evening skies of autumn to the brilliant stars of winter. We are beginning to face away from the center of the galaxy, looking at stars behind us in our own part of our galaxy (the Orion Spur).
Moon Phases in December 2015:
Last Quarter: Dec. 3, 1:40 a.m.
New: Dec. 11, 4:29 a.m.
First Quarter: Dec. 18, 9:14 a.m.
Full: Dec. 25, 4:11 a.m.
The Geminid meteor shower peaks every year around mid-December. This year, it’s the night of Dec. 13 (although they’re active from Dec. 4 to 17). Unlike most meteor showers, the Geminids are debris from an asteroid, 3200 Phaethon, rather than a comet. The path of debris thus intersects Earth’s at a much shallower angle than a comet’s path would. This means we turn into the path, and thus start seeing meteors, much earlier in the night than most showers. You can look for meteors as early as 10:30 p.m. If you’re far from city lights, you’ll see one or two meteors per minute on average. The three-day-old moon will not interfere. Our George Observatory will be open Sunday night, Dec. 13, until 2 a.m. for observing the meteors.
At 10:48 p.m. on Monday, Dec. 21, the sun will appear overhead as seen from the Tropic of Capricorn, the farthest point south where this is possible. That’s because Earth’s North Pole is tilted as far as possible away from the sun at that time. That’s why this is our winter solstice, the day when we have more night and less daylight than any other. Below the equator, this is the summer solstice because the South Pole is tilted towards the sun as much as possible.
Although the shortest day (or the least daylight) occurs Dec. 21, the earliest sunset occurs for us about Dec. 1. This is because the sun’s apparent positon in our sky varies like a sine wave; there is little difference in the sun’s apparent height for about a month before and after the solstice. Due to Earth’s tilt, the sun does indeed take a shorter, lower path across the sky on Dec. 21 than on Dec. 1, but only by a little. Meanwhile, Earth is slightly accelerating as it approaches perihelion just after the new year. This makes both sunrise and sunset happen a little earlier each night during December. Near the solstice, this small effect can dominate. Since most of us sleep through sunrise and watch sunset, days will seem to lengthen from Dec. 1 to 21 when they are in fact still getting shorter.
On most clear Saturday nights at the George Observatory, you can hear me do live star tours on the observation deck with a green laser pointer. If you’re there, listen for my announcement.