Few astronomical phenomena are as accessible to ordinary folk as a meteor shower. They usually get a few lines in the local papers, you don’t need any special equipment to see them, and they follow a predictable schedule which stays the same from year to year. The only problem is that they’re usually a lot less spectactular than people expect (The word ‘Shower’ is rather misleading), and you can easily get quite unpleasantly cold and stiff if you don’t know to take some basic precautions.
So what exactly is a meteor shower? A Meteoroid is a small chunk of stuff – rock, ice, whatever – out in space. They mostly range in size from the smallest mote of dust to an average sized pebble, although obviously stuff in space comes in all sizes (Like, for example, asteroids and planets). If a meteoroid is going to collide with Earth, it will first have to plough through the atmosphere at tens of thousands of kilometers per hour. At that speed, the atmosphere in front of the meteoroif is compacted and compressed in a shock-wave, so that the air itself glows white hot (and also emits radio waves – short-wave radio listeners will be familiar with the sudden spikes of noise produced by meteors). It’s so hot that it breaks down into a plasma which heats and melts the meteoroid. The popular understanding that the heat of re-entry is caused by friction with the atmosphere is a false one. At this stage, when the meteoroid is glowing white hot and smashing through the atmosphere, it is called a Meteor. If it is large enough to make it to the ground before burning up completely, it becomes a meteorite. Scientists treasure these as samples that can be studied to understand the rest of the The Solar System while gem collectors value their rarity.
Meteorids are scattered all over the Solar System, but they are especially concentrated around the orbits of comets. As comets pass the Sun, they heat up and their ices melt, spraying out vapour and dust. Some of this outgassing carries away larger pieces of debris, but since none of it travels very fast, it tends not to stray too far from the original comet’s orbit. When Earth passes through one of these regions, a great many more get scooped up into the atmosphere, and we see a meteor shower. Meteor showers vary in the brightness and speed of their meteors, as well as in the number seen per hour and the duration of the shower, depending on how close Earth’s orbit comes to that of the comet, and on the properties of the comet itself
There are a lot of meteor showers known. Some are so thin that they cannot easily be distinguished from the normal sporadic meteors – the random junk populating the solar system which isn’t a part of any shower, while others produce some of the most spectacular shows you could imagine – I have a childhood memory of a holiday with a church youth group when the sky lit up with bright strong meteors flashing by every few seconds, and have yet to see anything on earth or in the sky to match the glory of that show – clearly an unusually good one! These showers are carefully catalogued, and the debris clouds causing them are researched and measured so that astronomers can make detailed (and sometimes even accurate!) predictions of how many objects are expected to strike over that period.
Of course, for a casual amateur Urban Astronomer like myself, this is all very interesting, but in the end it’s just about a beautiful show, and if you can share it with a few interested friends, then so much the better! But unless you’re lucky enough to experience what I did at that youth camp, you’ll need to make a few preparations. First, know where to look. Because the showers are caused by Earth charging through a stream of debris, each meteoroid hits the atmosphere at the exact same angle. Thanks to perspective, this makes them all appear to radiate outwards from the same point in the sky (which is the place the Earth is moving towards at that exact point in its orbit). Find out where the current shower’s radiant is and face generally in that direction for best results. Second, arrange to be lying down – either flat on the ground, or in a reclining deck chair. You’re going to be staring at the sky for a long time, and if you aren’t in a good position you’ll suffer some vicious cramps in your neck before the night is over. Third, dress up warm. Even in the middle of summer, your body will lose a significant amount of heat from lying motionless for several hours under the night sky. Finally, realise that the published rates for a shower are in meteors per hour. You can’t see all of them, since you can’t see the entire sky at one, so your personal rate will likely be a little less. Even the busiest showers, with rates over 100 per hour translate to maybe a few meteors each minute. So manage your expectations, go in with the right mindset and some good company, and you’ll enjoy one of the most repeatable shows in nature.
The Arietids meteor shower, as it’s name suggests, has its radiant in the constellation Aries. It lasts from late May till early July and peaks between the 7th and 8th of June. The meteoroids of this shower enter the atmosphere at speeds of about 38 kilometers per second, and are usually “earthgrazers”, meaning we see them hitting the atmosphere at a shallow angle, ploughing slowly right across the sky. But most interestingly of all, the Arietids are a daylight shower. …Continue reading →
Meteor showers have their origins in comets, and the eta Aquarids come from the most famous of all: Halley’s Comet. Every time it swings past the Sun, the comet heats up and sprays particles of dust out along its orbit and twice a year the Earth blunders straight into the debris streem. When these bits of grit hit the atmosphere, they flare briefly before burning up and we see a meteor. The first time we pass through the stream each …Continue reading →
Like all comets, Halley’s Comet leaves a diffuse trail of dust and gravel in space, following along in the same orbit. Twice each year, Earth passes through this trail and as the various bits of debris collide with our atmosphere and briefly flare before burning up, we get a meteor shower. The first of these showers is called the Eta Aquarids and happens in March, while the second happens in October and is called the Orionids. The first meteors of …Continue reading →
The Perseid meteor shower is a very long-lasting shower, running for almost an entire month and radiating from the constellation Perseus. People have been watching the Perseids for about 2000 years, and the shower is sometimes referred to as the Tears of St Lawrence, since the shower was at its peak when he was martyred by the Romans in the year 258 AD. Individual meteors from this shower are particularly fast moving, entering the atmosphere at speeds of over sixty …Continue reading →
The Delta Aquarid meteor shower is actually two showers radiating from points within the same region of sky, and which overlap on the calendar. The Northern shower runs from July 16 to September 10, peaking on 13/14 August, while the Southern delta aquarids cover the period from July 14 to August 18 and peak on 29/30 July. These showers favour us in the Southern Hemisphere, partly because their radiants are high up in our skies, and partly because they peak …Continue reading →