★ Ursid Meteor Shower

The Ursids are the last major shower of the year, peaking just before Christmas, when many people are busy and the nights are long and cold. That is a pity, because their radiant never sets for most northern observers, and now and then they surprise everyone with an outburst.

Where the meteors come from

The Ursids come from Comet 8P/Tuttle, a periodic comet that circles the Sun every 13.6 years. Its orbit stretches from about Earth's distance from the Sun out to just beyond the orbit of Saturn. Over many returns the comet has left trails of dust along this path, and Earth passes through the stream each December. The meteoroids hit the atmosphere at about 33 km/s.

Oddly, many of the strongest Ursid displays happened when the comet itself was far away, near the outer end of its orbit. The dust trails do not simply travel with the comet, which makes outbursts hard to predict.

When to watch

The shower is active from December 17 to 26 and peaks around December 22. The maximum is fairly narrow, and its timing seems to shift a little from year to year, so watch on the nights of December 21–22 and 22–23. The table above gives the expected peak time and the Moon.

From mid-northern latitudes the radiant never sets. It is lowest in the evening, about 25 to 30 degrees up in the north, and climbs to about 50 degrees by dawn. That makes the hours before dawn the most productive, although you can see Ursids all night.

How to find the radiant

The radiant is in Ursa Minor, the Little Dipper, a couple of degrees from Kochab, the brighter of the two stars at the end of the dipper's bowl. Follow the two pointer stars of the Big Dipper to Polaris, then look for Kochab, an orange star about as bright as Polaris. In the evening the radiant sits below Polaris, low in the north; through the night it swings around to the right and climbs, and by dawn it is up and to the right of Polaris. The radiant does not rise at all for most southern observers, so this is a Northern Hemisphere shower.

What to expect

In an ordinary year the ZHR is about 10. With the radiant about 45 to 50 degrees up before dawn, a dark site gives about 4 to 7 Ursids an hour, and the suburbs 1 to 3. Some years bring more: IMO notes that the rate occasionally climbs to a ZHR of 50. Ursids are medium-speed meteors at 33 km/s, and with a population index of 2.8 they are a mix of faint and moderately bright meteors.

Viewing tips

• Face north or north-east and look about halfway up. The radiant is never overhead, so many meteors rise from the northern horizon.
• Dress for some of the coldest nights of the year: insulated boots, a hat, gloves and a sleeping bag.
• If the Moon is bright (see the table above), keep it behind a building or tree and concentrate on the northern sky.
• Send your counts to the IMO. The Ursids are poorly observed, and outbursts may have been missed in cloudy years.

History and notable displays

William F. Denning recorded meteors from near Kochab in the early twentieth century but saw no special abundance. That changed on December 22, 1945, when observers at Skalnaté Pleso Observatory in Slovakia saw a strong outburst, which Antonín Bečvář reported as a new shower. Zdeněk Ceplecha later showed that the meteors' orbit matched that of Comet Tuttle. A second major outburst came on December 22, 1986, with a ZHR of about 120. Esko Lyytinen and Peter Jenniskens later explained such events as encounters with old dust trails from the comet, and their prediction for 2000 was followed by a ZHR of about 90. IMO lists lesser enhancements from 2006 to 2008 and in 2011, 2014, 2015, 2017 and 2020.