Witnessing the bright streak of light as a meteor crosses the night sky is an exhilarating experience that has inspired many people to further their interest in astronomy including our own Dr John Mason.
Meteors occur when tiny fragments of interplanetary dust enter the Earth’s atmosphere at high speed and are rapidly heated up to become incandescent before finally burning up. Stray fragments of debris surround the Sun entering the Earth’s atmosphere so frequently that on a clear night with an unobstructed view of the sky, you will see one meteor every 15 minutes or so on average. These meteors known as sporadics and appear randomly in any part of the sky.
Meteor showers occur when the Earth passes through a particularly dense region of dust, left behind by a comet or asteroid, the number of particles entering the atmosphere can become very great indeed. This encounter creates an effect similar to what you observe when you drive through falling snow and the flakes seem to radiate from a single point. Meteor showers take their names from the constellations from which they appear to radiate. For example the Leonids, a shower that occurs each November appear to emanate from the constellation of Leo and the Perseids radiate from a point in the constellation of Perseus.
The measure of the number of meteors observed is a figure called the Zenithal Hourly Rate or ZHR which is the estimated number of meteors a single observer would see in one hour under a clear, dark sky if the shower radiated directly overhead i.e. the zenith. The ZHR of the Perseid shower is typically around 100 for the short period around the shower’s maximum.
Many showers such as the Perseids which take place each August, are annual events occurring when the Earth reaches the point in its orbit that it crosses the path of a comet. In the case of the Perseids the parent comet is called Swift-Tuttle.
Observations made over many years, mainly by amateur astronomers of both meteor showers and their parent comets has given us the chance to map the regions of dust through which the Earth passes enabling experts to predict when our planet might pass through a particularly rich region of debris leading to a spectacular shower. With each year, as more data is gathered, the models become more accurate but forecasting remains difficult and ultimately the only way to know whether the meteor shower will be a magnificent display is to go out and witness it for yourself.