Spend more than a few minutes under a clear dark sky and it will not be long before you see one of nature’s own fireworks, a meteor. Often known as shooting or falling stars, they are fun for beginners to amateur astronomy to study because no equipment is needed.
These flashes are simply pieces of cosmic debris blazing a fiery death as they vaporise about 100 km above the Earth. We now know that this most of this debris is the dust shed by comets as they cross the interplanetary spacelanes. Some is from comets long lost which we cannot identify. Other displays may be linked to comets that still appear in our skies today.
What might seem a bright streak is usually caused by just a tiny speck, no bigger than a grain of sand. As its zips through our atmosphere at up to 70 km per second, it ionises the air around it causing the brilliant glow. Sometimes this glow will produce a long-lasting ghostly train for several seconds.
Technically, the particle itself is called a meteoroid and the streak in the sky is a meteor. Meteoroids from comets almost invariably are completely destroyed in their blaze of glory. Objects termed meteorites are stones that have survived a fall to Earth and are more likely to be linked to the asteroids. A space rock big enough to reach the ground will produce a fireball in the sky.
Meteors may appear at random, in any part of the sky and at any time. However, there are particular periods in the year when we can predict a higher level of meteor activity. These are dates when, each year, the Earth crosses the orbit of a stream of dust left by a comet. The result for observers is what called a meteor shower.
Shower is something of an exaggerated term because from any one spot, you will rarely see more than one or two a minute at best. The number you see will depend not only on how much dust is entering the atmosphere but also on how dark your sky is and whether there is bright moonlight, for example.
Meteors that cannot be linked to a shower are called sporadics. They can be told apart from members of a shower because the shower meteors will appear to radiate from a particular point in the sky. No matter in which part of the sky you see the meteor, you can trace back its path and it will cross that point. The meteors appear to have shorter paths the closer they appear to the radiant because they are heading more directly towards you.
The effect is one of perspective. The meteors are actually flying parallel to each other as they stream into the atmosphere but traced back, their paths converge on their radiant just as the parallel lanes of a long straight motorway appear to converge in the distance.
Different showers are usually named after the constellation in which the radiant appears to lie. So a strong shower that occurs every August and appears to radiate from the direction of Perseus is called the Perseid meteor shower, or simply the Perseids. Similarly a major December shower radiating from Gemini is known as the Geminids. Some shower names even specify a bright star close to their radiants such as the δ Aquarids or κ Cygnids.
During a meteor shower, it is not only the sky brightness that affects the numbers seen. Many radiants don’t rise above the horizon until after midnight and the higher the higher the radiant is in the sky, the more meteors will be seen. Also, as we get closer to dawn, the part of the Earth where the observer is situated is likely to be turning into the direction from which the meteors come. The combined speeds of the rotating Earth and the travelling meteor particle make any meteor travel faster and burn brighter.
Meteor observing is an ideal project for the starting-out stargazer because little more equipment is required than a garden chair to sit in and watch the sky. It is a fun activity to share with friends but can be of real scientific value. You will need a watch, a notebook, a red light and a pen or pencil. A voice recorder is especially useful as it will allow you to make your notes without taking your eyes off the sky. (Make sure the batteries are fully charged and keep a spare). Find a spot well away from streetlights, sit for an hour or so at a time steadily watching a patch of sky, say half way between the horizon and the zenith. There is no need to watch the radiant.
When a meteor appears, quickly note the time, estimate the meteor’s approximate brightness compared to the stars, their colour, relative speed (e.g. fast, medium or slow) and any particular features such as whether they left a train. Note too whether the meteor was a shower member or a sporadic. You should note, too, the sky conditions such as the presence of any haze, cloud or moonlight and the brightness of the faintest stars you can see overhead, termed the limiting magnitude.
Your observations, properly made, will be very useful to major astronomical organisations and professional meteor scientists when combined with other members’ records.
Do not expect to see the number of meteors quoted for any shower’s maximum rate. This is usually a figure that would apply in ideal conditions and with the shower’s radiant is directly overhead. It is called the Zenithal Hourly Rate or ZHR.
There are very rare occasions when observers do witness great numbers of meteors. They are called meteor storms and happen when the Earth passes through a dense clump of particles in the stream’s orbit. The Leonids generally reveal just a handful of meteors at maximum in November but many thousands an hour may be seen at intervals of around 33 years. The last spectacular display was in 1999.