This week, two annual astronomical events will join forces to create a celestial double feature, as the Ursid meteor shower peaks during the winter solstice for northern sky-watchers.
Seasons (and solstices) happen because our planet spins on an axis that is slightly tilted. During winter in the Northern Hemisphere, Earth’s axis tilts away from the sun. On the day of the solstice, the sun takes its shortest path in the sky and hugs the horizon during its daily trek, making it the shortest day of the year.
At the same time, south of the Equator, the axis is tilted toward the sun, and people there are experiencing the height of summer. For sky-watchers in these regions, the December solstice is the longest day of the year.
The exact date and time of the December solstice changes slightly from year to year because of the difference between a calendar year of 365 days and the solar year of 365.26 days—the exact time it takes for Earth to make one trip around the sun.
This year, the December solstice officially occurs at 5:44 a.m. ET (10:44 UTC) on Wednesday, December 21.
Once the solstice sun sets, the fun really begins for sky-watchers: A minor meteor shower is set to peak across northern skies later in the evening and into the predawn hours of Thursday, December 22.
The sand grain-size particles that slam into our atmosphere to create the Ursid shower were shed many centuries ago from the comet 8P/Tuttle. Every year like clockwork, sometime between December 17 and 23, Earth plows through the debris stream left behind by this icy visitor.
The Ursids were first noticed in Europe back in 1900, and they are known to produce about 10 to 15 shooting stars an hour, on average. Because of the shower’s relatively low rates and cold winter timing, observing records have been sparse. Still, some keen stargazers have been surprised by rare bursts of 30 to a hundred meteors an hour.
As with all meteor showers, the Ursids are named after the constellation from which they appear to radiate—in this case it’s Ursa Minor, the little bear. Meteors will seem to shoot out from a region just to the left of the bowl of the Big Dipper, which will hang low in the northern sky for mid-latitude observers.
While the Geminids last week were mostly washed out by a superbright full moon, the Ursids promise to put on a decent sky show well before the half-lit moon rises around 1 a.m. local time.
Observers will have their best chance to catch the most meteors falling after local darkness sets in on the night of December 21. After moonrise, expect to see more modest meteor numbers flying across the skies—somewhere around five to 10 an hour.
So if you are headed out to do any last-minute holiday shopping this week, you may get the chance to make a wish on an Ursid shooting star as it falls from the heavens.