The Ursids, of the night of December 21/22, are the last noticeable meteor shower of the year.
The expected date of this shower’s modest peak is just a day after the midwinter solstice (of Dec. 21 at 10:44 Universal Time). And whereas the Geminids of Dec. 13/14 coincided with Full Moon, the Ursids come a day and a quarter after Last Quarter (Dec. 21, 1:56 UT). So the Moon, with its glare less than half as strong as when it was Full, will not rise into the sky till about an hour after midnight.
These meteors, also called Ursa-Minorids, appear to radiate from the constellation of the Little Bear, which has the North Pole Star at one end and Kokab at the other. So this radiant is in the sky all night, swinging around the Pole and becoming highest at the night’s end.
The Ursid meteors are particles following roughly in the orbit of the comet from which they crumbled: 8P Tuttle (the “P” denotes “periodic”), which last came by in January 2008 and is due next in August 2021. The orbit drops steeply from the north across the December part of Earth’s orbit, and that is why these midwinter meteors appropriately come to us from the Arctic of the sky – arktos is Greek and ursa Latin for “bear.”
They will probably be more of a light snow-flurry than a shower. At their peak, and under ideal conditions, only around 10 per hour may be seen. Yet there have been occasional Ursid storms, even to more than 100 per hour, as in 1993.
The rate is usually above half maximum for about half a day, and the peak this year is expected to be about 9 Universal Time on Dec. 22, which is convenient for America – 4 AM in the Eastern time zone, 3 in the Central, 1 on the west coast. Meteor streams can be clumpy and braided, and there could be subsidiary peaks in the two following nights.
Sky & Telescope has a good piece on the Geminids and Ursids in the December issue (pages 48-49). I notice that they’re now copying my style of representing meteor radiants in charts – well, you can’t copyright such things, and it won’t be the first time we’ve borrowed ideas from each other. I’d be interested to know whether the meteor streaks are drawn by hand in their art department or programmed, as I do, to relate to the strength of the shower and the nearness in time of the picture to the shower’s peak.
The Moon, before it reached Last Quarter, passed Regulus on Sunday Dec. 18, at 18 UT, so closely that it occulted the “Little King” star – but only as seen from the southernmost coast of Australia.