Brian Jones: The Perseids
Meteors – or shooting stars – appear as rapidly-moving streaks of light seen against the background of stars. They are caused by tiny particles of dust, of which there are billions orbiting the Sun, wandering close to our planet and being pulled down into the atmosphere as a result of the Earth’s gravitational attraction. Their entry speeds can be anything up to 45 miles (70km) per second which results in violent collisions with air particles. The resulting friction causes them to burn up and leads to their appearance as shooting stars.
Broadly speaking, there are two types of meteor. Sporadic meteors can appear at any time and anywhere in the sky. However, the appearance of shower meteors is much more predictable. These are associated with comets, and the particles that give rise to the meteors we see originate in material which has been thrown off by the parent comet as it passes through the inner solar system. This material initially goes to form the coma and tail of the comet. However, because the gravitational pull of the comet is very weak, the particles break away and eventually become spread out all along the orbital path of the comet.
The Earth’s orbit around the Sun carries it across the orbital paths of numerous comets at different times of the year. When this happens, and because the cometary orbit is littered with particles, a larger than average number are drawn into our planet’s atmosphere, which in turn results in relatively high numbers of meteors. Many meteor showers are fairly weak and produce only a small rise in meteor activity. However, the Perseid shower, produced during July and August when the Earth crosses the orbital path of Comet Swift-Tuttle, is the most famous and by far the most active of all.
Because the particles associated with comets travel around the Sun in parallel paths, the meteors they produce will all appear to emerge from the same point in the sky, an effect similar to that seen with railway lines which are also parallel but which seem to merge in the distance. This point is known as the radiant and the meteor shower is usually named after the area of sky in which the radiant lies. For example, the Perseid radiant lies in the constellation Perseus as shown on the chart below.
WATCHING THE PERSEIDS
Meteor watching is fairly straightforward and involves nothing more than laying under a starry sky making sure that (in spite of it being summer) you’re wrapped up warmly. The night of maximum activity for the Perseids is 12th August at which time the Earth is passing through the densest part of the cometary particle swarm. On that night anything up to 50 or more naked-eye shooting stars may be seen per hour. This year, the Moon will have set by early evening and so, assuming the sky is cloud-free, conditions for watching the Perseids are quite favourable.
If you spot a Perseid meteor it will first appear some distance from the actual radiant. Any meteor you do see can be confirmed as a Perseid by tracing its path back. Perseids will be seen to radiate from a point in the sky to the top (north) end of the constellation Perseus which, during evenings in early August, will be visible in the north eastern sky, a little way below the prominent W-shaped Cassiopeia and above the bright star Capella. The Perseid radiant is shown on the chart and it is from this point that the Perseids will seem to emanate. Those meteors whose paths don’t trace back to this point are sporadic meteors and not from the Perseid shower.
So, if you happen to be out walking on the night of 12th August – or indeed between now and then – keep an eye out for the Perseids… happy meteor-hunting!