The Ursids meteor shower
Key facts
- Peak
- around 21–22 December
- Active
- about 17–26 December
- Rate at peak (ZHR)
- about 10 (occasional outbursts)
- Speed
- 33 km/s
- Radiant
- Ursa Minor (near the star Kochab)
- Parent body
- Comet 8P/Tuttle
Dates shift slightly from year to year. This year's peak is in the events calendar, and the meteor-shower planner shows how high the radiant climbs from where you live, night by night.
The Ursids close the meteor year. They peak around the winter solstice, the longest night of the year in the northern hemisphere, and they are often overlooked, coming so soon after the spectacular Geminids and in the busy days before Christmas. That is a pity, because they can be a pleasant surprise.
The shower's radiant lies in Ursa Minor, the Little Bear, close to the star Kochab, one of the two bright stars at the end of the Little Dipper's bowl. Ursa Minor contains Polaris, the Pole Star, so the radiant is very far north and never sets as seen from Britain.
History. The shower was noticed in the early twentieth century by the English amateur astronomer William F. Denning, a tireless meteor observer working from Bristol. For decades afterwards it received little attention. Then on 22 December 1945 the Czech astronomer Antonín Bečvář and colleagues saw an unexpected outburst, with rates of more than 100 an hour. Further outbursts have been recorded since, including one in 1986, showing that the Ursids, like several other showers, can occasionally surprise.
The parent comet. The Ursids come from Comet 8P/Tuttle, which takes about 13.6 years to orbit the Sun. It was first seen in 1790 by Pierre Méchain and rediscovered in 1858 by the American astronomer Horace Tuttle, who also gave his name to the parent comets of the Perseids and Leonids. Ursid outbursts seem to happen when the Earth passes through denser dust trails left by the comet on earlier orbits, and some have occurred around the times the comet itself is far from the Sun.
What to expect. In a normal year the ZHR is about 10, so you might see 5 to 10 Ursids an hour under dark skies. They enter the atmosphere at about 33 km/s, a moderate speed, and are mostly faint. The peak is fairly narrow, lasting about a day, so it is worth checking the predicted maximum.
Viewing from the UK. The UK is well placed. Because the radiant is circumpolar, Ursids can be seen all night. The radiant is lowest in the north in the evening and climbs higher towards dawn, so the best time is in the hours before morning twilight. The Little Dipper is fainter than the familiar Plough (Big Dipper), so a dark site helps. Find Polaris first, using the two pointer stars of the Plough, and then look for Kochab nearby.
Viewing worldwide. The Ursids are a northern hemisphere shower. From most of the southern hemisphere the radiant never rises, and from the tropics it stays very low.
Why so far north? A radiant near the celestial pole means the Ursid stream meets the Earth from high above the plane in which the planets orbit. The meteoroids are therefore not ploughing head-on into the Earth, nor catching it up from behind, but arriving at a steep angle, which helps explain their moderate speed. It also means the radiant never sets from Britain: it circles Polaris, lowest in the evening and highest around dawn.
December nights are long and cold, and the Ursids often coincide with frost. Wrap up well, and consider combining an Ursid watch with a look at the winter constellations such as Orion and Gemini. Peak dates vary slightly from year to year.
Viewing tips
- Find Polaris using the pointer stars of the Plough, then look for Kochab nearby.
- Watch in the hours before dawn, when the radiant is highest.
- Check the predicted peak, because maximum activity lasts only about a day.
- Choose a dark site, because most Ursids are faint.
- Dress for a frosty night and bring a blanket or sleeping bag.
Read about the parent: Comet Tuttle, and the other comets behind meteor showers.