This year, once again, we are about to pass through a stream of meteoric material.
Most of it is made up of dust and sand grain-sized particles of rock or ice, but there are some larger lumps in the stream. The particles are all moving in the same direction and at the same speed, around 60 kilometres a second — or about 216,000 km/h.
When they hit our atmosphere, friction against the air heats and then vaporizes them. The atoms making up the air have no time to move out of the way, so they pile up in front of each particle, heating up to about 50,000 degrees. What we see in the sky is a glowing streak or trail made up of hot, ionized air and evaporated particle.
The bigger lumps can produce more dramatic displays. As they move through the atmosphere at high speed, they tend to erode and melt into an aerodynamically stable shape, like a sphere or pancake, which gradually vaporizes away.
However, if the stresses cause part of the body to break off, it goes into a violent hypersonic tumble that causes it to disintegrate. We see the streak across the sky terminate in an explosion.
We often hear that the time to see the Perseids is the night of August 12. This is partly true — that is the night the Earth passes through the core of the stream.
The stream is actually quite broad, so we can start looking for Perseids any time after mid- July through to the end of August. However, the best time is roughly between August 9 and 14.
In theory the best time of day is between midnight and dawn, because that is the time our part of the Earth is facing in the direction we are orbiting the Sun, “looking through the windshield.”
In the evening, we are looking behind us, “through the rear window” and we always collect many more bugs on the windshield than we do on the rear window. We will still see some meteors though, because they are moving fast enough to overtake us.
Since all the meteoric objects are moving in parallel paths, they appear to us as though they are all radiating from one point in the sky, which happens to lie in the same direction as the constellation of Perseus, hence the name.
On evenings this time of year, Perseus lies fairly low in the northeastern sky, so a view in that direction will show most meteors, but in general, the more sky you can see, and the darker it is, the more meteors you will catch. This means getting away as far as practical from our street and house lights.
Lights have two bad effects. First, if they are shining in your eyes, apart from being annoying, they stop them achieving the sensitivity needed to enjoy the sky. Second, there is usually some haze in the air. This gets illuminated by all those lights, whether you can see them or not, making the sky brighter and harder to see faint objects.
Get away from town if you can.
Unless you are lucky to be looking in the right direction at the right time, binoculars or telescopes are close to useless for looking for meteors. They see such a tiny patch of sky the chances of one passing through that patch while you’re looking at it is minimal.
Your unaided eyes are the best tools, so the big issue is being comfortable and warm so you can just relax and take in the sky. A chaise longue or deckchair is ideal. Otherwise, use a blanket on the ground.
It gets cold if you are sitting still for long periods, even on summer nights. Make sure you have a blanket to lie under. Plan on spending at least an hour. Hot chocolate helps.
It won’t be boring. As your eyes get dark-adapted and your brain gets tuned in, you will see more and more stars, some wisps of cosmic gas and dust, satellites wending their way across the sky, and of course, some Perseids.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory located west of Okanagan Falls.