Every year in late April, those of us who watch the night sky are treated to the Lyrid meteor shower. Not to be confused with the Leonid shower that occurs in November, this shower has been viewed longer than any other, with its first recorded observation over 2,600 years ago.
A meteor is a fast-moving streak of light that will last for only a second or two as it crosses the sky. A meteor shower occurs when the number of meteors we see increases for a period of a few nights. The period of the Lyrid shower is approximately April 16 to 26, with the peak around April 22. It’s not a strong shower, so you can expect to see 10 to 20 meteors per hour. However, the shower could surprise you with outbursts of 100 or more meteors per hour.
Most meteors are caused by particles the size of peas that enter the Earth’s atmosphere moving at thousands of miles per hour. The streak of light we see is not the particle itself, but the reaction of the gases of the atmosphere to the high temperatures created by the friction of the high-speed particle and the atmosphere. Most meteors burn up in the atmosphere at altitudes of 60 to 80 miles.
The particles, called meteoroids when they are in space, have many sources. They can be asteroids or pieces of asteroids in random orbits around the sun. Even debris blown off of the moon or planets from impacts with other meteors can create meteoroids.
The meteoroid that created the fireball over Russia on Feb. 15 was most likely an asteroid. But the meteors that come from these sources are random and do not create the recurring annual meteor showers.
The source of annual meteor showers is debris from comets. In the 19th century it was discovered that comets shed particles every time they pass close to the sun. These released particles continue to move around the sun in the same orbit as the comet. Because of gravitational interactions with other objects in the solar system, they spread out through the entire orbit of the comet.
There are some comets whose orbits intersect or pass very close to the orbit of the Earth. At these points in the Earth’s orbit, the Earth will pass through these clouds of particles, creating a meteor shower. The density of the particles in the cloud determines the number of meteors we see. Low density produces few meteors while a higher density produces more.
The parent comet of the Lyrid shower is C/1861 G1 Thatcher. It is a periodic comet with an orbital period of 415 years. This comet has not been seen since its discovery in 1861 by amateur astronomer A. E. Thatcher of New York. Until 1983, this was the only long-period comet to be linked to a meteor shower.
The orbit of this comet is inclined about 80 degrees to the plane of the solar system, so it passes the orbit of the Earth at nearly a right angle. In 1861 the comet was within 30 million miles of Earth, but at its farthest point it will be 110 AU (astronomical units) from the Earth. (One AU equals the average distance between the Earth and the sun.)
The radiant of the shower, the point in the sky where all of the meteors appear to originate, is in the constellation Lyra; hence its name, Lyrid shower. In April Lyra rises in the northeast around 10 p.m., but this year an almost-full moon will light up the sky at the peak of the shower.
Because of the moonlight, it will be even more necessary to view this shower from a dark site away from town. The greatest number of Lyrid meteors often falls in the dark hours before dawn; this year the best time is predicted to be in the few hours between moonset and dawn on the morning of April 22.
The unpredictability of the rate of the Lyrids makes this a shower to watch. If you are out at night between April 16 and 26, take a little time to look for the Lyrids. Who knows? This might be the year of another outburst.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at email@example.com.