On Friday night last week, I set out with the express intention of staying up late to watch the Orionids.
I was disappointed.
We had thick cloud cover and heavy rain. The sky might have been beautifully dark, but I couldn’t see a single star in it, let alone a shooting one.
The rain and cloud continued into Saturday. In the afternoon, it started to clear a bit, and I thought I might be lucky, but it clouded over again before it got dark.
Luckily, my cats thought that Saturday night was an excellent time for a game of “let’s charge like mad things all around the house” and “oh look, a trampoline!” Suffice to say, when large cats repeatedly jump on you, you wake up! Normally, I’d be quite annoyed by this, but on Saturday night (or the wee small hours around about 4am on Sunday morning) I wasn’t, because I realised I could see stars out of the window: the sky had cleared!
I wrapped up warm, and spent a peaceful hour or so sitting in my back garden watching the Orionids (and fiddling with camera settings to try and get a reasonable picture: I succeeded with the settings, but not with a picture actually showing them).
The Orionids are a meteor shower which occurs annually in mid-October, when Earth passes through the trail of dust and debris left behind by Comet Halley. The peak days for viewing the Orionids this year, when they were most intense, were the nights of the 20th and 21st of October. These coincided with the moon being just a thin sliver, so light from it was not obscuring the view of the meteors.
Shooting stars are undoubtedly exciting, and staying up late to watch for them can be a rather fun thing to do. You don’t need any special equipment – just a clear (dark) sky and a bit of patience. (A warm coat or sleeping bag, a chair, and some hot chocolate are also good to have, especially in winter!) Sit back and look at the sky – they’re often very fast, so you need to stay focused, otherwise you miss them.
But what are shooting stars? Where do they come from? Why do we get ‘meteor showers’ every so often?
Shooting stars, or meteors to give them their proper name, are bits of dust, rock, and space debris which enter Earth’s atmosphere and burn up. Outside Earth’s atmosphere, they are known as meteoroids, and if they reach Earth itself, they become meteorites. When they enter Earth’s atmosphere, they are travelling really fast – up to about 45 miles per second – and this is what causes them to glow.
As the meteor crosses into Earth’s atmosphere, it pushes the particles of the atmosphere closer together, compressing them. This creates a localised pocket of increased pressure right in front of the meteor, which in turn increases the temperature.
To understand this, we need to think about particles (like we did last week – particles, as a concept, really are very important). Imagine a sealed container of a gas – the particles are spread out and at random. As they collide with the sides of the container, they exert a pressure. If you increase the temperature, the particles move more and faster, so they collide with the sides of the container harder and more often, and the pressure increases. This works both ways though – increasing the pressure will also increase the temperature.
The meteor is travelling really fast, so the increase in pressure is huge, so the temperature increase is huge, effectively creating a sort of ‘personal oven’ for the meteor. The intense heat and the oxygen cause the meteor to burn up. What we see as a ‘shooting star’ is the combination of this burning up with the meteor’s descent through the atmosphere – a streak of light across the sky. Friction actually has very little to do with it – the forces just aren’t big enough.
Our solar system is made up of the sun at the centre, with the planets radiating out, orbiting the sun. Between Mars and Jupiter, you also find the asteroid belt. (The asteroids are lots of lumps of rock, of varying size. Some of the dwarf planets we know about also orbit in the asteroid belt.) All those things that orbit the sun have circular orbits – the sun is at the middle, and they trace a path around it to make a circle.
We also have a number of comets in our solar system. Comets are made of ice, dust, and rock, and also orbit the sun, but in an elliptical (oval) shape with the sun at one end. Comets have a tail made of dust and gas (actually, two tails, one of each), which leaves a trail of debris behind the comet. When Earth crosses through the debris left in the path of the comet, some of it inevitably enters the Earth’s atmosphere. Because there is so much of it in a relatively small space, we get a lot of meteors in a short time period – a meteor shower. We know the orbits of comets, and we know Earth’s orbit, so we can predict fairly accurate when meteor showers will be from where the orbits cross (and they’re at about the same time each year).
If you missed the Orionids, don’t worry! There are more meteor showers this year, and they all repeat again in 2018.
The Leonids will be active between November 5th and December 3rd, peaking around November 18th, when Earth crosses the orbit of Comet 55P, Tempel-Tuttle.
The Geminids will be active between November 30th and December 17th, peaking around December 13th. These come from asteroid debris, rather than a comet – Phaethon.
The Ursids will be active between December 17th and December 24th, peaking around December 22nd, when Earth crosses the orbit of Comet 8P, Tuttle.
They’re all named after the constellation nearest which they appear – but if you look at the sky you should be able to spot some; you don’t need to know where the constellation is. (Binoculars don’t help – they restrict your field of vision.)