Look up with the December 2017 sky map to navigate the stars and constellations in the night sky. On this page is both a color sky map and a black and white printable map to bring outside!
Just click here or on the image below to open the printable map—then bring outside!
This month’s highlight: A Hot Shower on a Chilly Night
If you gaze at the sky long enough on any clear, dark night, you will eventually see a few meteors, popularly known as “shooting stars.” These are tiny particles of fast-moving cosmic debris that—purely by chance—encounter Earth’s upper atmosphere. The particles are moving so fast, relative to Earth, that friction with air molecules causes them to heat up to melting point. Within a fraction of a second they vaporize in a flash of light that we see as a thin glowing streak: a meteor.
A meteor or two is a welcome bonus if you are already observing planets or constellations. However, if you want to see lots of shooting stars, you need to observe a meteor shower, an event that occurs just a few times a year. This is one of those times: The best meteor shower of 2017 falls on the night of December 13–14.
The Geminid Meteor Shower (aka the Geminids) gets its name from the point in the sky (the radiant) from which the meteors appear to originate. The radiant for the Geminids lies (surprise!) in the constellation Gemini, the Twins. Although all of the Geminid meteors fly away from the radiant, the shooting stars may appear in any part of the sky. This month’s Sky Map shows the radiant for the Geminids and illustrates how the meteors can appear anywhere.
Meteor showers occur when Earth encounters a particularly dense stream of cosmic debris. The size of the debris is on order of dust particles or grains of sand. Most meteor-shower debris streams are produced by comets. For example, debris left in the wake of Halley’s Comet is responsible for two meteor showers, the Eta Aquarids in May and the Orionids in October.
The Geminids, however, does not have a cometary origin; in fact, the parent body responsible for the shower remained a mystery until 1983. In that year, the 3-mile-wide asteroid Phaethon was discovered. The source of the Geminids had been found! Ordinarily, asteroids don’t produce much dust and debris, but something happened to Phaethon a few centuries ago that caused it to begin shedding copious quantities of dust. Every December, Earth plows into the stream of debris left behind by Phaethon, and the happy result is an excellent meteor shower.
You might think that the best way to observe the Geminids would be to look directly at the radiant. You would be wrong! Although all of the Geminids will appear to be flying away from the radiant, they will not necessarily appear near the radiant. Again, the meteors can appear anywhere in the sky! The best strategy to maximize the number of meteors you see is to the look toward the darkest and least obstructed part of the sky that is observable from your location. The darker your surroundings, the better. You can begin looking up any time after it’s truly dark, but the peak hours are usually after midnight.
With luck you’ll average about one meteor a minute from a good location. They will not arrive like clockwork, though. You might go several minutes without seeing anything, then be treated to a few meteors all at once. So dress warmly, bring a hot drink, and look for particles of asteroid Phaethon as they meet their fiery end high above Earth.
December Sky Map
Click here or on image below to enlarge this map (PDF).
Sky map produced using Chris Marriott’s Skymap Pro
Enjoy astronomy? Check out Bob Berman’s column, “This Week’s Amazing Sky.”