Meteors, Meteorites & Meteoroids

Also known as shooting stars, meteors are very small pieces of rock which enter the Earth's atmosphere at high velocities (typically 40km/s)They are seen as a bright streaks across the sky as they ionise the surrounding air. The larger the piece of rock, the brighter the meteor. If a meteor appears above magnitude -4 in the sky then it is termed a fireball. Some of the larger pieces of rock may land on the Earth - they are called meteorites. When a piece of space rock has not yet collided with the Earth, it is called a meteoroid. Meteoroids are the debris left behind fom comets. In 1772 Biela's comet was discovered. It was seen every 6 years there after, until 1842 was it was seen to have split in two! The pair returned in 1852, were missed in 1859 because they were badly placed, and failed to appear at the expected return of 1866 - in fact they have never been seen since. When they should have returned in 1872, a brilliant meteor shower was seen coming from the part of the sky where it should have been. There is no doubt that the "Andromedid" meteors of 1872 singnalled the death of Biela's Comet. The meteors were seen again in 1885, 1892 and 1899 but now to all intents and purposes the shower is extinct.

above. A Drawing of Biela's Comet made in 1846.

above. This drawing was made by Otto Wilhelm Struve (Pulkovo) on 1852 September 25. It shows both pieces of comet 3D/Biela, with component B being the brightest. North is toward the top right.

Now a number of other showers can be seen at different times of the year. The strongest are the Perseid meteors which can be seen radiating from the constellation of Perseus around 11th August.
The first part of the table below shows the details of major meteor showers throughtout the year, and the lower part displays some of the weaker showers.

Each meteor shower has its own "radiant" the is the constellation from which the meteors appear to come from. For example the Geminids of December appear to come from Gemini. The Quadrantid meteors radiate from the constellation of Bootes, but they are called the Quadrantids because when they were discovered, the top part of Bootes was called Quadrans (the Quadrant). The Quadrantids have a very well defined sharp peak, so if this happens during daylight, the best part of the shower may be missed. The date shown for each shower is the approxomate date of the peak, but most of the showers extend before and after that by about two weeks. Some showers on the other hand are only visable for a short amount of time (such as the Leonids - November 15 -November 25) and others have a very large span (such as the Northern Taurids September 19 - November 30). The moon is the meteor watcher's worst enemy as the number seen will be greatly reduced. The last column of the table gives the ZHR. This is the "Zenithal Hourly Rate" i.e. the number of meteors you could expect to see if the radiant was at the zenith (the point directly above an observer) and if the moon was not present. This rarely occurs so the number is usually less. More meteors shold be seen after midnight because it is then that the Earth is facing directly into the meteoroid stream.
When going on a meteor watch, you should look 45 degrees from the radiant and 45 degrees from the horizon. Most observers find it most comfortable to use a deck chair for meteor watching.
Many meteor showers have their own characteristics. For instance the Perseids of August have many meteors that explode and my leave "trains" which are trails of smoke that can be seen for a few minutes afterwards and the Taurids of November have many bright fireballs 3 days after the maximum.
Some meteors do not belong to specific showers - these are called sporadics. On any night of the year one should expect to see around 8 sporadic meteors.

The picture above is "meteor crater" in Arizona. It is a crater that is 1265meters (4150 feet) in diameter and 175meters (575 feet) deep, formed by a meteorite that crashed to the ground in prehistoric times. It is well formed, and has been described as "The most interesting place on Earth". The exact age of it is unknown, as the present estimate of 20,000BC may be to young. Although the meteorite came in at an angle the crater is circular. The table below shows some of the most important meteorite craters.