What's Up - December 2020

What's Up for December? A meteor shower, an extraordinary close meetup between Jupiter and Saturn, and the winter solstice.

December brings one of the most reliable annual meteor showers - and one of the best in 2020: the Geminids. This shower is active from December 4th through the 17th, as Earth plows through the trail of dusty debris left behind in the orbit of asteroid 3200 Phaethon - which might actually be a burnt-out comet.

The Geminids produce a good number of meteors most years, but they're made even better this year as the shower's peak coincides with a nearly new moon. (Thus making for darker skies, with no moonlight to interfere with the fainter meteors.) The Geminids peak overnight on December 13th into the morning of the 14th, with some meteor activity visible in the days before and after. Viewing is good all night for the Northern Hemisphere, with activity peaking around 2 a.m. local time, and after midnight for viewers in the Southern Hemisphere.

For the best viewing, find a safe location away from bright city lights, lie flat on the ground with your feet pointing south and look up. Meteors can appear in any part of the sky, though they'll appear to radiate from near the constellation Gemini. So here's wishing you clear skies to catch some shooting stars.

Jupiter and Saturn have been traveling across the sky together all year, but this month, get ready for them to really put on a show. Over the first three weeks of December, watch each evening as the two planets get closer in the sky than they've appeared in two decades. Look for them low in the southwest in the hour after sunset. And on December 21st, the two giant planets will appear just a tenth of a degree apart - that's about the thickness of a dime held at arm's length! This means the two planets and their moons will be visible in the same field of view through binoculars or a small telescope. In fact, Saturn will appear as close to Jupiter as some of Jupiter's moons.

This event is called a "great conjunction." These occur every 20 years this century as the orbits of Earth, Jupiter, and Saturn periodically align making these two outer planets appear close together in our nighttime sky. Even so, this is the "greatest" great conjunction between Jupiter and Saturn for the next 60 years, with the two planets not appearing this close in the sky until 2080.

The 21st is also the date of the December solstice, which is the winter solstice in the Northern Hemisphere and the summer solstice in the Southern Hemisphere. On the December solstice, the Sun reaches its southernmost position in the sky, no matter where on Earth you happen to be.

In the Northern hemisphere, the Sun travels its lowest, shortest path across the sky on that day. Thus, in the north, the winter solstice brings the shortest day of the year, in terms of hours of sunlight.

Now the Sun's changing height in the sky throughout the year is caused by Earth's tilt as it orbits our local star. The tilt causes the amount of sunlight each hemisphere receives to go up and down in the annual cycle of the seasons.

Here are the phases of the Moon for December. You can catch up on all of NASA's missions to explore the solar system and beyond at nasa.gov. I'm Preston Dyches from NASA's Jet Propulsion Laboratory, and that's What's Up for this month.