Meteors

Barringer Meteor Crater - photo by B.P. Snowder

When a large meteor strikes it can blast out a crater, like one did 25,000 years ago: the Barringer Crater near Winslow Arizona. About 1.2 km across and .2 km deep, the meteor that caused it is estimated at 50 meters diameter which would have weighed about 300,000 tons.

The Barringer meteor was very small compared to the object that created the Chicxulub Crater in the Yucatán 65 million years ago. That is the one that is theorized to have led to mass extinctions. It is estimated to have been 6 to 15 kilometers in diameter.

Meteors are sometimes called "shooting stars" or "falling stars." Most are only bits of gravel the size of your fingernails or smaller. They are the leftover debris from the formation of the solar system. Many are part of rivers of debris scattered by passing comets. If the Earth passes through one of these rivers of debris, we have a meteor shower. Occasionally the Earth passes through a very dense area of the river, and we have a meteor storm with hundreds of thousands of streaks per hour.

"On the night of November 12-13, 1833, a tempest of falling stars broke over the earth... the sky was scored in every direction with shining tracks and illuminated with majestic fireballs. At Boston, the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. Their numbers ... were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much-diminished rate, that 240,000 must have been visible during the nine hours they continued to fall." ~ Agnes Clerke

Engraving of the 1833 Leonid Meteor Storm

When the Earth runs into a patch of debris that is all moving parallel, like the stuff left by comets, it looks like the streaks radiate outward from a particular point in the sky. This happens for the same reason that parallel train tracks appear to converge to a point on the horizon. It's just perspective. The streaks in each of the major meteor showers appear to radiate from a point in a particular constellation as the Earth reaches a certain point in its orbit each year. This is what gives the names to the showers. When a meteor happens during a shower but doesn't radiate from the point like the rest, it is called a rogue - not a true member of the stream. On the other hand when a true member of the stream comes days earlier or later than the rest of the herd, it is called a stray.

The best time to see meteors is between midnight and dawn. That's when you are on the part of the Earth that is "in front" as it travels through space. In fact, at dawn you are on the very bow of the ship.

On an average day about 4 billion meteors enter our atmosphere, totaling more than a hundred tons. Debris enters the earth's atmosphere and gets heated by friction because it travels at such high speed, 10 to 70 kilometers per second. For a few seconds they streak across the sky. The brightness we see is actually the hot ionized gasses surrounding the object. Usually the object completely burns up before hitting the ground, but some are large enough to survive and impact the ground. These are called meteorites.

Sometimes a meteor will enter the atmosphere weighing several kilograms. these are spectacular events called fireballs. The glowing trail following fireballs sometimes continues to glow for up to 30 minutes. Fireballs are typcially the brightest objects in the night sky. If you see a fireball, remember to take a quick look at how distinct your shadow is. Also, listen carefully, sometimes fireballs explode. Exploding meteors are called bolides.

Major Meteor Showers
Shower	Peak Dates	Meteors/Hr/Observer	Associated Object
Quadrantids	Jan. 2-4	40-100+	unknown
Lyrids	Apr. 21-22	10-15	Thatcher
eta Aquarids	May 4	20	Halley
delta Aquarids	July 29	15-20	unknown
Perseids	Aug. 11-13	50-70+	Swift-Tuttle
Draconids	Oct. 9	6	Giacobini-Zinner
Orionids	Oct. 21	25	Halley
Taurids	Nov. 3-5	15	Encke
Leonids	Nov. 17	10-20	Temple-Tuttle
Andromedids	Nov. 25-27	5	Biela
Geminids	Dec. 14	50-80+	3200 Phaethon
Ursids	Dec. 22	5	Tuttle

Zodiacal Light
This is a pyramid of dim light seen in the west after evening twilight or in the east after morning twilight. It is light scattered from the Sun by countless micro-meteoroidal particles along the plane of the solar system. The best time to look for the subtle effect is under a moonless sky, in late February or late September, when the ecliptic is at a steep angle with the horizon. Look carefully to the west on a late February evening just as twilight is ending, or in the east in late September just before sunrise.

Gegenschein
German for "counter-glow," this is caused by micro-meteoroidal material in space at the anti-solar point. By refectling sunlight they are acting as millions of tiny "full moons." Like zodiacal light, the effect is best seen in late February or late September under moonless skies.