December Solstice via Solar Walk 2 app

Happy Holidays, everyone! Or, as we astronomers say, “Have a Happy Solstice and a Merry Perihelion!”

(Above: As the Earth orbits the sun, it’s polar axis points continuously at the same point in the sky. On the June and December Solstices, the northern hemisphere’s pole tilts directly towards and away from the sun respectively, causing the noon-day sun to reach its highest and lowest points in the sky for the year. A lower sun delivers weaker solar radiation for a shorter number of hours, giving us cold winter temperatures.)

For the Northern Hemisphere, the first day of winter, also called the Winter Solstice, occurs on Thursday, December 21st at 11:28 am Eastern Time. At that precise moment, the north pole of the Earth’s axis of rotation will be tilting directly away from the Sun. Every day, at local noon, the Sun reaches its highest position in the sky for that day. But at the Winter Solstice, that position is the lowest (i.e., farthest south, celestially) for the entire year, and we receive the shortest amount of daylight. The sunlight that we do get this time of year is diluted because it’s spread over a larger area, the same way a flashlight beam looks dimmer when you shine it obliquely at a wall (try it!).

Fewer hours and weaker sunlight both translate into less received solar energy (insolation) and therefore colder temperatures! Good news for us, though — after Thursday, our days start growing longer again! For our friends in the Southern Hemisphere, the Sun will attain its highest noon-time height for the year on the solstice, and it marks the start of their summer season.

It is NOT the case, as some people think, that we are colder in winter because we are farther to the Sun (a position called aphelion) — in actuality that happens every year in early July! On the contrary — we’re approaching Earth’s nearest position from the Sun (perihelion), which occurs every January 4, or thereabouts.

Above: The annual Ursid Meteor Shower peaks on December 22 before dawn. Meteors can appear anywhere in the sky, but all Ursids will appear to travel away from the radiant above the Little Dipper.

Ursid Meteor Shower Peak

The annual Ursid meteor shower runs from December 17 to 23 annually. This year, it peaks during the wee hours of Friday, December 22, when seeing 10- 20 meteors per hour is possible under dark skies. The best time to watch will be from midnight until dawn that morning, but you can also watch after dinner. The moon will be a young waxing crescent that sets in early evening on the peak night, setting up good meteor watching conditions. The shower’s radiant is above the Little Dipper in Ursa Minor (Little Bear), near Polaris, but the meteors can appear anywhere.

The Moon and Planets

The New Moon phase, when the moon is crossing the imaginary line between Earth and the Sun, occurs at 1:30 am on Monday morning. So the moon will be hidden from view beside the Sun in the daytime sky. The first chance to see the moon again comes Monday at sunset, when its extremely slim silver crescent will be low in the western sky. Over the following few evenings, the moon will wax (grow thicker) and draw away from the Sun, but it will still set early in the evening — giving us dark moonless nights for most of this week.

If you’re an early riser, you’ve probably been seeing a very bright object shining in the eastern dawn sky. That’s Jupiter, which rises about 4 am local time this week and is well above the southeastern horizon by dawn. Much dimmer and red-tinted, Mars, which rises before Jupiter, at about 3:30 am local time, is sitting to Jupiter’s upper right. On Monday morning, Mars will be about a fist diameter from Jupiter. But over the course of this week, Mars’ eastward orbital motion will carry it towards Jupiter, so the separation between them will shrink to a palm’s width next weekend.

Above: The pre-dawn sky, shown here for 6:45 am local time on Decmeber 24, 2017, has all the naked eye planets this month — with Mars rising first, followed by Jupiter, and then Mercury. Over the course of this week, Mercury’s orbit swings it away from the sun, making it easy to spot about 6:45 am local time

Mercury re-appears in the pre-dawn sky this week. It’s going to be quite easy to see — especially later in the week when it has climbed farther from the sun. Between about 6:45 and 7 am local time, it will be the medium bright “star” sitting low in the southeastern sky.

With Venus and Saturn hidden near the sun, Uranus and Neptune are the only planets left in the evening sky, setting about 2:30 am and 11 pm local time respectively. Blue-green Uranus is midway between the two chains of stars that form the dim constellation of Pisces (the Fishes). Tiny blue Neptune, only observable in a backyard telescope, is about half a finger’s width below the medium-bright star Hydor in Aquarius (the Water-Bearer).

Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note. Astronomy Skylights for this week (from December 17th, 2017) by Chris Vaughan.