During the night of 12-13 August, the Earth will pass through the debris stream filling the orbit of Comet Swift Tuttle. Thousands of small pieces of that debris, ranging in size from dust upwards, will crash into our atmosphere at tens of kilometres a second. Friction will slow them rapidly, dissipating their kinetic energy as heat. The air surrounding the debris fragments will be heated to thousands of degrees and the particle vaporized. It is rare for any of these debris particles to reach the ground. What we will see are transient bright trails of incandescent air and vaporized debris forming in the sky and vanishing as they cool. We commonly call these trails “shooting stars”. However tiny bits of debris are nothing like stars; the correct name for them is “meteors”.
The Solar System is filled with these fragments of material, left over from its formation, so we can see meteors on any clear night, particularly if we are lucky enough to be well away from city lights. However, on nights like 12-13 August, when we will speed through a stream of fragments crossing in front of us at many kilometres a second, we will see a much larger number of meteors. We call these “meteor showers”, and name them after the constellation we see in the distant background when we look up the meteor stream. In the case of this shower the constellation is Perseus, so we call it the Perseids.
Imagine you are sitting between the rails on a railway track. Don’t try this; just imagine it. Ahead we see the rails passing on either side of us and converging to a point in the distance. The railway tracks are the same distance apart, but appear to be converging. Similarly, we see the meteors coming from more or less the same point in the sky and fanning out, although they are all moving parallel to one another.
To enjoy the event, find a place shielded from street and other lights, better still, a place out of town with a good view of the sky. At this time of year Perseus lies in the northeastern sky, so we will see the meteors streaming out of the northeast. The Moon will be waning and rising late, so observing conditions could be good. You will need a blanket to lie on, or a chaise longue, so you can look up for long periods without strain. Have a blanket, because even on August nights you can get cold.
For something a little different, you can pick up meteors with an FM radio. Just tune it to a distant station that cannot normally be picked up. In Western Canada, Sunny 102.3, Modesto, California works well. You will hear bursts of signal as it echoes off the meteor trails. If you do this while lying on a blanket looking at the stars, use headphones, so that everyone can enjoy the event in their own way. Radio counting is best between midnight and noon, when our side of the Earth is facing in the direction of the meteors.
This year, in the early hours, the planets Jupiter and Venus rise spectacularly in the eastern sky, and are an additional treat, especially if you have binoculars or a small telescope.
Mars, Saturn and the star Spica are close together low in the west after dark. Venus and Jupiter lie close together in the eastern sky in the early hours. The Moon reaches last quarter on the 9th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astro-physical Observatory, Penticton.