Meteors and Meteorites

On any clear night of the year an observer can expect to see about 10-12 sporadic meteors per hour. Sporadic meteors can appear from any part of the sky, and about 500,000 sporadic meteoroids enter the Earth's atmosphere every day.

Meteor activity is often described in terms of the number of meteors observed per hour. The observed hourly rate of meteors will be dependent upon the prevalent "seeing" conditions, and factors such as the presence of a full moon, local light pollution, and Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. clouds will reduce the meteor count and hence lower the observed hourly rate. Astronomers often quote a corrected hourly rate which describes the number of meteors that an observer would see, each hour, if the observing conditions were perfect.

Observations have shown that the corrected hourly rate of sporadic meteors varies in a periodic fashion during the course of a day. On a typical clear night the hourly rate of sporadic meteors is at a minimum of about six meteors per hour at 6 P.M. The hourly rate climbs steadily during the night until it reaches a maximum of about 16 meteors per hour around 4 A.M.

This daily variation in the hourly rate of sporadic meteors is due to Earth's rotation in its orbit about the Sun. In the evening, a sporadic meteoroid has to catch up with Earth if it is to enter the atmosphere and be seen. This is because at about 6 P.M. local time an observer will be on that part of Earth's surface which is trailing in the direction of Earth's motion. In the early morning, however, the observer will be on the leading portion of Earth's surface, and consequently Earth will tend to "sweep up" all the meteoroids in its path. An observer will typically see two to three times more sporadic meteors per hour in the early morning than in the early evening; and will see them at high speeds relative to Earth.