There are so many things to see in the sky this the month, including exciting planetary line-ups, two meteor showers, and a supermoon! The Northern Cross is also upright in the west in the evenings, in time for Christmas.
All times in Eastern Standard Time, for the Northern Hemisphere:
December 3 — The December Cold Moon will be astronomically full at 10:47 a.m. In this phase, the visible Moon is fully illuminated by direct sunlight. Though the Moon is only technically in this phase for a few seconds, it is considered “full” for the entire day of the event, and appears full for three days. This Moon will also be considered a supermoon because it coincides the closest with perigee (December 4). Of the year’s 12 full moons, only December’s full Moon comes close enough to Earth (222,443 miles or 357,987 km) to be considered a true supermoon. Our only one of 2017!
Learn more how this month’s full Moon got its names in this short Farmers’ Almanac video:
December 4 — The nearly-full waning gibbous Moon is at perigee, its closest point to Earth in its orbit.
December 7 — Earliest sunset of 2017 at 40º N. latitude. This comes come some 2 weeks before the winter solstice, not on the solstice as you might think!
December 8 — Look eastward after midnight to see the waning gibbous Moon paired up with the star Regulus. Regulus will be sitting above the Moon.
December 10 — Last quarter Moon, 2:51 a.m. In this phase, the Moon appears as a half Moon in the sky. One-half of the Moon is illuminated by direct sunlight while the illuminated part is decreasing, heading toward the New Moon (invisible) phase.
December 11 — Check out the southeast sky about 90 minutes before sunrise this week. That orange-yellow “star” below the waning crescent moon on Wednesday morning is Mars. During December Mars rises about 4½ to 4 hours before the Sun, and can be readily observed about one-third up from the southeastern horizon at the break of dawn.
December 11-13 — Bundle up for the annual Geminid Meteor Showers! These showers will peak on December 13th. Normally one could expect up to see up to 120 meteors hourly with this display, but the Moon’s brilliant light will likely obliterate all but the very brightest meteors (Wait till next year!). Regardless, they’re considered the best meteor showers of the year and it’s worth taking a look. The radiant — that spot in the sky where the meteors will appear to emanate — lies just below and to the right of the bright star Castor in the constellation Gemini (hence the name, “Geminids”). Best viewing after midnight when the radiant point is high in the sky, until dawn, no matter where you are. You might even see an earthgrazer!
December 18 — New Moon at 1:30 a.m. At this stage, the Moon is not illuminated by direct sunlight and is completely invisible to the naked eye. Incidentally, this new moon ushers in the longest lunar month of the 21st century (2001 to 2100). A lunar month is the period of time between new Moons.
December 18 — The New Moon is at apogee, its farthest point from Earth in its orbit. Need an easy way to remember? (A)pogee = (A)way.
December 21 — Winter officially begins at 11:28 a.m. with the Winter Solstice. The Sun reaches its farthest point south of the celestial equator so it’s the shortest day of the year in terms of sunrise to sunset. The good news is that the days will start getting longer from here!
December 20-23 — Nature’s annual holiday light show, the Ursid meteor showers, are at their peak. Visible from the north all night, these meteor showers generally produce anywhere from 5 to 15 meteors per hour at their peak (usually on the first full night of winter, Dec. 22). They are the dusty debris left behind in the orbit of Comet Tuttle. There have been a few occasions when the Ursids have surprised observers with a sudden outburst many times their normal hourly rate (over 100 per hour in 1945), but such cases are very few and far between.
December 29 — Before sunrise, look to the southeast sky to see Mercury (hugging the horizon), Jupiter, higher in the sky, and Mars directly above Jupiter.
December 30 — The Moon occults Aldebaran, the brightest star in the constellation Taurus.