Meteor showers occur when the Earth passes a debris stream
left over by a comet, the streams can be dense or sparse leading to strong and weak meteor showers, some meteor showers are so weak as to be
almost imperceptable. Others are quiet strong and can lead to a good numbers of meteors.
When the Earth encounters one of
these debris streams, the meteors caused by them tend to look like theyre coming
from a single point in the sky, much like the effect you get when you stand on a bridge
looking down on a motorway, the cars appear to originate from a single point on the horizon, and then
either go left or right as they get
nearer. The point in the sky the meteors appear to be coming from is called the radiant. The constellation where the radiant lies
usually gives its name to that meteor shower, so the Perseid meteor shower has its radiant in the constellation Perseus. Any meteors
who's path does not trace back to the radiant are called "Sporadic Meteors", these are meteors that would have occured
irregardless of the meteor shower and are not associated with it. You would expect to see between 5 and
10 of these per hour on any given
night. The higher the radiant is, the more meteors you'll see in the meteor shower. Meteor showers are rated by a term call
"Zenith Hourly Rate" (ZHR). This is the estimated number of meteors you would see if the radiant was directly above you.
When observing meteor showers,
it is best to observe them after midnight, in the evening you are looking into
the sky where the Earth has already been. After midnight the Earth has rotated enough that you're
looking into the direction the Earth is moving into. Think of it like this, the Earth is a car you are sitting in,
you are looking out the rear window and after midnight you are looking out the front
window, you're going to see more bugs hitting the
front windscreen then the back window. You will see meteors before midnight, but you'll see more after.
Most meteors are only the size
of a grain of sand, ones the size of a big rock can survive the firey entry into
the Earth atmosphere. If it reaches the surface it is called a meteorite. Exceptionally bright meteors
are called "Fireballs". If a large rock/meteor hits the atmosphere and bounces back into space, it is called
a "Bolide" The
bright meteors can leave a persitant trail that can last several seconds.
A dark sky is best for observing
meteors as city lights will overwhelm the dimmer ones and most meteors tend to
be dimmer with a small minority been very bright. A large Moon in the sky is also a distinct
disadvantage. The club hold observing nights for the best meteor showers throughout the year,