2016 September 24
If you’re outside on a dark clear night for about an hour or more, the chances are that you will see one or two ‘shooting stars’ over that time. These objects can be bright and swift – rather like a celestial firework – but in spite of their name, they have nothing at all to do with the stars.
Meteors are the result of tiny particles called meteoroids (or micrometeoroids) in space colliding with the Earth’s atmosphere. They are very small, typically about the same size as a grain of sand. As they pass through the upper atmosphere, they collide with air molecules which causes them to heat up, and as the meteor burns away it leaves a bright streak called a trail. After a bright meteor has vanished very often you will see a ghostly track where it was – this may last a couple of seconds, and is called the meteor train. In contrast, meteorites are much larger objects that survive their passage through the Earth’s atmosphere, eventually reaching the ground.
Sometimes larger debris hits the atmosphere creating a bolide or superbolide. These are very bright, may cast shadows and even produce a sonic boom as they pass through the atmosphere. Objects larger than 10cm heat up due to ram pressure (pressure exerted on the object by the atmosphere) rather than friction. The International Astronomical Union currently defines a fireball as any meteor which is brighter than a planet – i.e. anything brighter than about magnitude -4 in the night sky.
Showers and sporadics
Space is full of debris, some of it manmade, but much of it originating from comets and asteroids. Comets are vast chunks of rock and ice which orbit the Sun, usually in rather elliptical orbits. They tend to spend much of their time in the frozen outer darkness of the solar system, but when they come closer to the Sun, they heat up, typically producing massive dust and ion tails. These tails deposit a lot of dust into space and if the Earth passes through this swarm of particles, we experience a meteor shower.
From our point of view on Earth, a shower takes the form of large numbers of meteors all apparently coming from one particular point in the sky. Most showers have a single night of maximum activity, where the largest number of meteors for that shower are observed, but activity also usually extends to either side of the date of maximum.
When the Earth passes through a debris cloud, this cluster of particles hits the atmosphere in parallel trajectories, but because we are on the ground a long way away from them, they appear to come from a single point in the sky – in much the same way that railway tracks appear to meet in the far distance. The railway tracks are of course parallel (as are the meteoroid trajectories) but the effect of perspective makes it seem that they meet at a distant point.
The apparent source of a meteor shower in the night sky is called the radiant, and the constellation the radiant sits in gives the name of the shower. For example, the radiant of the Perseids lies in Perseus, the radiant of the Geminids lies in Gemini and so on. We now know of many meteor showers and the objects responsible for them. The annual Perseid shower is brought about by our passage through material left by comet Swift-Tuttle, and the Geminids are the result of debris originating from the strange object (3200) Phaethon.
Not all meteors come in showers, and it is not unusual to see a few meteors in the sky during the course of any night. Meteors not associated with any known shower are called ‘sporadics’. During a meteor shower there may be a number of sporadics sighted which are not associated with the shower you are observing.
Meteor observing is a very simple activity, and does not depend on having a lot of experience under the night sky. The best time to begin observing meteors is during a active shower, such as the Perseids in August or the Geminids in December.
A lot of useful information can be obtained from meteor observations. For example, we can determine if the radiant of the shower has moved. If the shower is more or less active now than in previous years, we can learn something about the object which produced it. In Table 1 I have given a list of some meteor showers coming soon in the autumn and winter, with the dates during which they are active (a more comprehensive list can be found in the BAA Handbook).
A number you will often see quoted with meteor showers is the ‘Zenithal Hourly Rate’ (abbreviated to ZHR). Essentially, this is the predicted number of meteors which might be observed in an hour if the radiant was above your head (at the zenith) in a clear, dark sky.
Although useful to indicate which showers can be worth trying to observe, this number can be very misleading. For example, the Geminids have a quoted ZHR of some 100+ meteors. However, the ZHR you see very much depends on the height of the radiant in the sky, and is dramatically reduced when the radiant is lower down, as of course it usually is. The ZHR also assumes a very clear sky and that your observing site is very dark – any light pollution will affect the ZHR, indeed even the presence of the crescent Moon is sufficient to bring the ZHR down significantly!
On top of all that, remember it is only a prediction, so don’t be disheartened if you observe decidedly fewer meteors than the ZHR figure.
Meteor observing is one of the few branches of astronomy which requires no special equipment at all. The BAA Meteor Section website is a good place to go for tips on observing and what to record, but essentially some basic equipment would include:
- A comfy deck-chair or garden lounger, and blankets or a sleeping bag (even in the summer, it can get a little chilly if you’re lying still for a long period of time outdoors);
- An accurate clock (your mobile should be sufficient, but don’t have its brightness set too high or it will interfere with your night vision; look for an observing app that lets you turn the screen to red);
- A good star atlas (Norton’s) or planetarium software;
- A red torch (white light will destroy your night vision);
- A clipboard, pens and paper, or a portable recorder to note your observations;
- A chart of the night sky showing the radiant;
- Food and drink to keep you going in the early hours (although not alcohol for obvious reasons!)
If you’re observing a specific shower, you must make sure that you note meteors which are genuinely part of that shower; don’t confuse them with sporadics which will appear during the course of the night, although sporadics should be noted also. There is a simple way to tell if a meteor you’ve observed is part of the shower or not: firstly, it has to be travelling away from the radiant. Secondly, the further away from the radiant the meteor is, the longer its trail should be: Figure 2 illustrates this principle and shows four genuine Geminid meteors, but the other two are sporadics.
What to record
While meteor showers can be dynamic and exciting, you will encounter quiet periods where nothing much is happening, so you do need to be patient! You also need to stay alert, otherwise you can miss faint meteors.
The BAA Meteor Section has a standard form which you can find online, and you will need to print out a copy for use during your observations. Before the shower starts, fill in the details at the top of the form including your name, location, name of the shower you are observing and so on. One thing you will need to record is the ‘stellar limiting magnitude’ – this is the magnitude of the faintest star you can see, and provides an estimate for the faintest objects you can see with the naked eye from your site. It is easy enough to do; simply wait until your eyes are dark adapted (typically 15-20 minutes) and then record the magnitude of the faintest star you can see. It is best to use a well known constellation near the zenith (immediately overhead) whose stars you can easily identify. You will need to look up the apparent magnitude of the star and record this on your form as the limiting magnitude.
As the shower progresses you should note your observations in real time, either directly on the form or with a handheld digital recorder for transcribing later. Record the time (in BST if your phone/clock shows it, but later remember to subtract an hour for UT before sending in your observations), together with details of the meteor’s trail and train if any, the constellation(s) in which it passed and whether it was part of the shower or a sporadic. You need to estimate its magnitude, and this is simple enough to do: just compare it mentally with the various objects given in Table 2 (or again see the Meteor Section website). The Section website also has a sample form filled in from a 1995 Leonid watch, which will show you what a completed form should look like.
Once you’ve finished observing, and perhaps made a fair copy of your observation forms (most of us quickly develop our own private code in the heat of excitement during a busy shower, so this and your hurried scrawls will need translating!) do make sure you send in copies of your completed forms to the Meteor Section Director. There is absolutely no point in making observations and then doing nothing with them! Your results will form part of an ongoing narrative – a study which the BAA has been engaged in for over 125 years. Meteor observing is a very satisfying and rewarding pastime, and probably one of those few occasions where all that is really required are your eyes and a clear dark sky.
Paul G. Abel
1 BAA Meteor Section: https://britastro.org/meteor
2 BAA Meteor Observing form: https://britastro.org/pdf/MeteorSectionVisual ReportBlank.pdf
3 BAA Meteor Section completed form: http://www.britastro.org/images_old/report1.gif