This daily variation in the hourly rate of sporadic meteors is due to the Earth ’s rotation in its orbit about the sun. The hourly rate climbs steadily during the night until it reaches a maximum of about 16 meteors per hour around 4 AM. Minimum of about six meteors per hour at 6 PM. On a typical clear night the hourly rate of sporadic meteors is at a Observations have shown that the corrected hourly rate of sporadic meteors varies in a periodic fashion during the course of a day. 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. 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 clouds will reduce the meteor count and hence lower the observed hourly rate. Meteor activity is often described in terms of the number of meteors observed per hour. Sporadic meteors can appear from any part of the sky, and about 500,000 sporadic meteoroids enter Earth ’s atmosphere every day. On any clear night of the year an observer can expect to see about 10 –12 sporadic meteors per hour. The meteoroid ablation process typically begins at heights between 62 –71 mi (100 –115 km) above Earth ’s surface, and the whole meteoroid is usually vaporized by the time it has descended to a height of 43.5 mi (70 km).Īstronomers have found that the visually observed meteors are derived from two meteoroid populations a continuously active, but sporadic, background and a number of specific sources called meteoroid streams. The speed with which meteoroids enter Earth ’s atmosphere varies from a minimum of 7 mi/sec (11 km/sec) to a maximum of 45 mi/sec (72 km/sec). Visual meteors (shooting stars) are produced through the vaporization of millimeter-sized meteoroids. It is estimated that 22,000 tons (20,000 metric tons) of micrometeoritic material falls to earth every year. The smallest of meteoroids can safely pass through Earth ’s atmosphere without much physical alteration, and they may be collected as micrometeorites at Earth ’s surface. The size limit below which vaporization is no longer important is about 0.0004 in (0.01 mm). The amount of surface heating that a meteoroid experiences is proportional to its surface area, and consequently very small meteoroids are not fully vaporized in the atmosphere. Continued heating causes more and more surface mass loss in a process known as ablation, and ultimately the meteoroid is completely vaporized. As the meteoroid continues its journey through the atmosphere, its surface layers become so hot that vaporization begins. Some of the meteoroid ’s lost energy is transformed into light it is this light we observe as a meteor. These collisions will both slow the meteoroid down and heat its surface layers. Upon entering Earth ’s upper atmosphere, a meteoroid begins to collide with an ever-increasing number of air molecules. If a meteoroid survives its passage through the atmosphere, without being fully vaporized and falls to the ground, it is a called a meteorite. A visual meteor, or shooting star, is produced whenever a meteor as large as (or larger than) a grain of sand is vaporized in Earth ’s upper atmosphere. In astronomy, “meteor ” is synonymous with “meteoroid, ” defined as any solid object moving in interplanetary space that is smaller than a few meters in diameter. Today, the Greek term is also associated with the scientific study of weather (meteorology). The word meteor is derived from the Greek meteron, meaning something high up.
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