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While a close approach of a comet, particularly one as impressive as Halley’s, is a rare event, other visitors to Earth not only drop in more frequently, but reach our very front door. To see these visitors, wait for dark, moonless night. If you are able to see the Milky Way, conditions are just about right. After a short while, maybe five minutes or so, a white streak will suddenly flash across part of the sky – a shooting star. These flashes give the impression of a star slipping off its pedestal and sliding through the sky. Not only are shooting stare not actually stars, but they are just about the most unstar-like objects one can imagine. Stars are incredibly massive and able to easily swallow up all the planets of the Solar System; shooting stars, or meteors as they are called officially, are minute grains of dust.
The nearest star besides the Sun is many billions of miles away from us; shooting stars are less than 100 miles above the Earth’s surface. A shooting star is seen when a particle of interplanetary dust floating through space collides with the Earth’s atmosphere. The dust passes through part of the atmosphere at a speed of tens of miles per second and quickly burns up and emits a short burst of light. The dust particles are called meteoroids while still is space and meteors as they flare through the Earth’s envelope of air. Although we imagine most of interplanetary space as quite empty, there is enough dust up there to bombard the Earth from 20 to 30 million times a day. If each of these meteors produce a visible shooting star, it would be an incredible sight; most however, are far too small to be detected by our eyes.
Shooting stars don’t always come in the same shape and size. They have paths of different lengths and levels of brightness. Some move rapidly and others relatively slowly across the sky. Occasionally you might see a fireball, a meteor so bright that it lights up the entire landscape. About two or three times a day over the entire Earth, a meteoroid will be so large (about the size of a basketball) that it will produce a fireball and reach the surface somewhat intact as a meteorite. A fireball on its way to becoming a meteorite shines with as much light as a quarter Moon. Don’t worry about a meteorite falling on your head. Of the several that drop onto the Earth’s surface daily, most land in the sea or in other non-populated areas.
On any dark night you will see a few meteors. On a few exceptional nights, there will be a meteor shower – large numbers of meteors falling within a short time. During one extraordinary night in 1966, for a brief period, meteors fell at a rate of about 100,000 an hour. Such abundant meteor showers can occur at anytime without warning. Other less prolific but still impressive showers take place at fixed times during the year. For instance, around the end of October each year, there is a noticeable increase in the appearance of meteors. Not only does their number sharply rise, but they all seem to be shooting out from the same point of sky between the constellations Orion and Gemini. Similar events occur throughout the year. Meteors will be seen in large number and always in the same area of the sky. These meteor showers will have other characteristics that remain unvarying from year to year. Besides the day of peak activity, the speed and brightness of most of their meteors will be similar from one year to the next.
The appearance of each of the almost 20 annual meteor showers is related to a comet. The late October meteor shower called the Orionids is caused by an earlier apparition of Comet Halley. During an apparition, Halley happened to pass through the Earth’s orbit. As the comet moved through interplanetary space, it left a trail of dust and other small particles behind which continue to roughly follow the comet's orbit. As this trail of dust particles lies along the Earth's orbit, we must unavoidably collide into them on about the same date each year. While each dust particle may be floating tens or hundreds of miles from its neighbor, the Earth passes through them so rapidly that collisions may take place many times within a single hour. The dust particles that create a meteor shower are moving parallel to one another.
The point in the sky from which the shooting stars emanate is called the shower's radiant. The radiant is like a distant point on the horizon at which there is a railroad track. If you are standing at a continuation of the same track many miles away, the track will appear as two nonparallel lines emanating from the point on the horizon even though, in reality, the two tracks are exactly parallel to one another. Or better yet, you might imagine that your are being approached by a squadron of jets in tight formation. If they pass directly overhead, some will appear to veer to the right and others to the left as they pass by.
In any case, by knowing the location of an expected shower’s radiant, you will know where to look in the sky for the approaching shooting stars. The best time for looking at meteors is after midnight on a moonless night. Remember that the predicted hourly rate of meteors may rise above or fall below the predicted rate. During some year, the Draconids do not appear at all while on others they provide great display. Meteor watching is like going to a very undependable movie theater. Sometimes you will be disappointed; on others, you can sit back and enjoy a great show.