Of Particular Significance

Meteor Showers: Some Facts

Meteors occur anywhere on any night of the year. But sometimes there are far more of them. These increases in meteor rates are due to the Earth encountering a relatively narrow stream of small dust grains and small pebbles. We refer to the abundant meteors seen in these encounters as “meteor showers.”

During an active meteor shower like the Perseids or Gemenids, dozens of meteors may enter the atmosphere each minute; individual observers may see one or more of them per minute.

A Little Terminology

A visible meteor (the moving dot in the sky that often leaves a trail behind it) occurs when a stone (called a meteoroid), usually no larger than a small pebble, enters the Earth’s atmosphere roughly 100 km (60 miles) above the ground. As the meteoroid proceeds, friction from air molecules heats it to high temperature, causing the air around it to glow. This glow is what creates the bright dot seen from the Earth’s surface, and it remains visible until the meteoroid has largely vaporized, usually while still at a high altitude of at least 50 km (30 miles). [Occasionally, larger objects survive their fall and hit the Earth; but these meteorites are rare and not our focus here.]

A Basic Illusion

As illustrated by the photographs in Fig. 1, meteors traveling in parallel and arriving at Earth appear as though emerging from a “vanishing point”, just as is seen in straight railway tracks coming a distance. Meteors further from the radiant appear to have longer trails.

Figure 1: (Left) Parallel tracks appear to emerge from a vanishing point. (Right) Parallel meteors also appear to emerge from a vanishing point, called the “radiant” , as seen in this long-exposure, all-sky image. (Meteor image credit: Berkó Ernő)

This is evidence that meteor showers arise from a horde of small stones all following the same path. As noted in more detail here, Newton’s understanding of gravity implies that the meteoroids in a shower must all have essentially the same speed and direction, to a good approximation. Otherwise they would spread out rapidly, the stream of particles would diffuse away, and no meteor shower would be seen.

On average, one observes a difference between meteors after midnight compared to those before midnight. The cause is partly as illustrated in Fig. 2. Between your midnight and noon, you are located on the forward half of the Earth — that is, the half that points in the direction of the Earth’s motion around the Sun. (This can be inferred by watching the motion of the other planets, all of which move around the Sun in the same direction as the Earth.) So when it is after midnight but before sunrise, you are standing in the dark forward quadrant of the Earth. For reasons explained here, the majority of meteor showers have a radiant that is overhead after midnight rather than before.

Figure 2: As the Earth orbits the Sun (green arrow) it rotates so that the side of the Earth that is seeing sunrise is facing forward in the direction of Earth’s motion, while that seeing sunset faces backward.

Because of this, meteors in showers more often more often emerge from radiants that are overhead after midnight. For such showers, a purely geometric effect implies (see Fig. 3) that somewhat fewer meteors will pass over your head before midnight into your field of view than will do so after midnight. However, those fewer pre-midnight meteors, which strike the edge of the Earth and are called “Earth-grazers”, will often make longer trails, and their altitudes will decrease more slowly, then post-midnight meteors. They are particularly useful for measuring meteors’ speeds.

Figure 3: The meteors in a meteor shower all travel in the same direction. As seen from the red dot, the meteors’ tracks will seem to splay outward from a point overhead (left inset). By comparison, a person at the green dot will see fewer meteors with longer tracks. These are known as Earth-grazers, as they almost miss the Earth’s atmosphere, and they will splay from a point near the horizon (right inset.) In many showers, the radiant is overhead well after midnight, and in that case, the green dot is representative of one’s view before midnight, while the red dot is representative after midnight.

Sometimes it is incorrectly stated that pre-midnight meteors are slower than post-midnight meteors due to the “rain-on-the-windshield effect”, but this is not correct: all the meteors in a single shower have just about the same speed. (The “bugs-on-the-windshield” analogy is even more incorrect, because bugs move much more slowly than cars, while meteors move faster than the Earth.) What is true, however, is that meteors in meteor showers that arrive from pre-midnight directions are slower than meteors in meteor showers that arrive from post-midnight directions. The reason for this is explained on this webpage.


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