Ursid meteors peak in moonlight

Ursid meteor shower active each year around December solstice. In 2018, the peak is probably Saturday morning, December 22.

Ursid fireball! A fireball is just a very bright meteor. Eliot Herman in Tucson caught it on December 18, 2016.

Die-hard meteor watchers usually start watching for Ursid meteors around now, as this low-key meteor shower is active each year from about December 17 to 26. The Ursids usually peak around the December solstice, perhaps offering 5 to 10 meteors per hour during the predawn hours in a dark sky. This year, in 2018, the peak of the Ursid shower falls on the same date as the full moon, on December 22 – so the full moon is sure to put a damper on this year’s display.

But here’s what’s cool about this meteor shower. Some meteor showers, such as the Perseids in August, have been watched each year at the same time for many centuries. The Ursid shower is new. It was first observed around the turn of the 20th century, when a skywatcher noticed that some meteors seen around this time of year weren’t random in their direction of motion across our sky’s dome, but instead appeared to radiate from near the star Kochab in the bowl of the Little Dipper asterism.

Also, although the Ursid meteor shower has been observed for only a single century – and although rates are typically around 5-10 meteors per hour – the Ursids have gained popularity in recent years because of their possible outbursts. Bursts of about 100 meteors per hour happened in 1945 and 1986. An unexpected increase of 30 per hour came in 1973.

So, once again, meteor-watching is like fishing! You never know what you’re going to get, but you hope you catch a big one!

Ursid meteors radiate from very far north on the sky's dome, near the Big and Little Dippers.

Ursid meteors radiate from very far north on the sky’s dome, near the Big and Little Dippers. This chart shows them around midnight, or when the Big Dipper is well up in the north-northeast.

All meteors in annual showers have radiant points on our sky’s dome, and the showers take their names from the constellations in which the radiant points lie. The Little Dipper asterism is in the constellation Ursa Minor the Lesser Bear. Hence, the Ursid meteor shower.

Ursa Minor is the constellation that contains the North Star, or Pole Star, called Polaris. From that alone, you know it’s a far-northern constellation. For this reason, the Ursids aren’t easily visible to observers in the Southern Hemisphere; their radiant point is just too far north.

If you want to try watching the Ursids in 2018, find a country location where you can camp out. Dress warmly! And plan to spend several hours reclining under a sky free of artificial lights, sometime during the night of December 21 (morning of December 22).

At very northerly latitudes – say, latitudes like that of the northern U.S. and Europe – the radiant of the shower is out all night. Should you try observing on the evening of December 21, or the evening of December 22? Up to you. Just know that the radiant point of the Ursid shower rises upward throughout the night, and reaches its highest point for the night just before dawn.

By the way, meteors in annual showers nearly all stem from comets (the Geminids being a bizarre exception). Beginning around the 1970s, astronomers found evidence that the Ursids move in the same orbit as Comet 8P/Tuttle, which is now believed to be the meteors’ source.

No matter what time of year you look, the two outer stars in the Big Dipper’s bowl always point to Polaris, which marks the end of the handle of the Little Dipper.

Bottom line: We anticipate the Ursid meteor shower will peak in 2018 on the morning of December 22, though under the glaring night of the full moon.

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EarthSky’s meteor shower guide

Bruce McClure