Observing Meteors

Introduction

Most people at some time or other have probably seen a “shooting star” dash across part of the sky. This of course is not really a star at all, but is a minute speck of cosmic material called a meteoroid, which “burns up” during its passage through the Earth’s atmosphere, producing the streak of light which we call a meteor. This webpage looks at what meteors are and some of the ways in which we can observe them. The illustrations used here have all been taken from the printable PDF booklet version of “Observing Meteors“.

What Are Meteoroids?

Meteoroids are the particles that produce meteors when they encounter the upper layers of the Earth’s atmosphere. The particles that produce naked-eye meteors are mostly very tiny, ranging in size from roughly a few tenths of a millimetre across, up to a few centimetres, or occasionally larger. These particles orbit the Sun much as do all other moving bodies in the Solar System, travelling at high speeds relative to the Earth, with velocities between about 11 and 72 km/sec. This means that even the rarefied atmosphere at heights of around 60 to 110 km above the surface is dense enough to cause the particles to ablate (“burn up”) owing to frictional heating by collisions with the air molecules. The ablation occurs very quickly. A meteor usually lasts for only a fraction of a second, or perhaps a second or two in rarer cases. Physically, most meteoroids seem to have an open or porous structure, and are very light, “fluffy” and friable – a little like coffee granules. The calculated average density is very low at about 0.3 grammes per cubic centimetre, roughly the same as cork.

All of these features have a bearing on how bright a meteor any given meteoroid will produce, but generally speaking the size or mass of the body is of greatest importance, with larger or heavier grains yielding brighter meteors. The smaller grains are by far the more numerous, however, with meteoroids large enough to produce fireballs (a fireball is any meteor of magnitude -3 or brighter) being rare. From regular visual observations, only about one meteor in every 150 is this bright, while a magnitude -8 fireball occurs on average about once in every 2000 meteors, for instance!

Each year, roughly 40,000 of these particles are large enough to survive their atmospheric flight and reach the Earth’s surface. If recovered, we call them meteorites. Most fall into the sea and are lost, but the few rescued are of three chief categories, determined by their chemical compositions: 1) Stones – rich in silicate minerals like quartz; 2) Irons – magnetic and contain nickel; 3) Stony-irons – transitional between the other two classes. These groups may well be similar in chemistry to many “ordinary” meteoroids too, as some meteor spectral analyses have suggested.

Origins

Meteoroids are thought to originate in asteroids or comets, though some may be remnants from the early days of the Solar System. Particles could also be liberated from asteroids by collisions, but many asteroids (particularly those like the Apollo asteroids which have orbits that pass close to, or cross over, the Earth’s) may actually be “extinct” or degassed cometary nuclei.

Comets emit large amounts of gas and dust, especially when close to the Sun, and while the gas will be swiftly removed by the effects of the solar wind, the dust will be rather less affected on a short timescale, and will tend to remain concentrated fairly close to the comet’s orbit. On a longer timescale, however, this dusty stream will gradually spread away from the original orbit and become dispersed owing to the effects of the Sun’s radiation and planetary gravitational perturbations. This gives rise to the majority of randomly-distributed meteoroids.

In the case of a single asteroidal collision or perihelion passage for a long-period comet, events which may happen only once in several thousand years for any single body, gravity and solar radiation will act to scatter the dust away from its source orbit before a repeat event can happen. Meteoroids thus produced will also end up with essentially randomly-distributed orbits.

If a repeated series of low-velocity asteroidal impacts occurs, such as might happen after the catastrophic destruction of one asteroid by another, or if a comet has a short orbital period (less than about 200 years), then the dust emitted will tend to remain near the parent body’s orbit, as fresh material will be deposited on each new orbit to compensate for that which is gradually lost. This concentration is relative only, as the particles in even a very dense meteoroid stream are each separated by several kilometres. These streams too will show signs of spreading and dispersion as time passes, and it is possible, to some extent, to determine the relative age of a stream by examining its physical appearance.

Sporadics and Showers

The randomly-distributed meteoroids give rise to the sporadic meteor flux as seen from Earth. Sporadics can be observed on all nights throughout the year, and can occur at any time, coming from any direction.

Meteoroid streams produce meteor showers when the Earth intersects their orbit. Shower meteors thus appear only at certain times of year, and can be told from the sporadics in that their paths, if traced back across the sky in dead straight, but imaginary, lines, seem to meet in one part of the heavens which is called the shower’s radiant. The shower is then named after the constellation or star near which this radiant lies, hence the Perseids seem to radiate from Perseus, the Geminids from Gemini and so forth.

The radiant effect (illustrated in the “O” of “Observing” at the top of this webpage, where lines representing meteor streaks appear to be emanating from the Perseid radiant) is seen because shower meteors travel towards the Earth on roughly parallel paths. Perspective makes these parallel lines seem to radiate away from a point, just as railway tracks or motorway lanes apparently meet at a “vanishing point” on the horizon when seen from a bridge over them.

Meteor Activity

The number of meteors observed over a given time – the rates – will vary depending on the time of night, the time of year, the sky clarity, the observer’s eyesight and, for shower meteors, the elevation of the radiant. Few shower meteors can be expected when the radiant is low in the sky: none at all when it is well below the horizon!

Sporadic activity varies daily (diurnally) and over the course of the year (annually). In order to define these changes, we can make use of the uncorrected mean observed hourly rate (OHR), the actual average number of meteors a good observer might see in reasonable skies, but the mean computed hourly rate (CHR), which corrects this rate to perfect-sky conditions, is more useful. The CHR is almost always somewhat higher than the OHR.

Both sporadic variations are due largely to the position of the Apex of the Earth’s Way on the sky. This is the direction in which the Earth is travelling through space. It is the point on the ecliptic 90° west of (“behind”) the Sun. The effect produced is similar to a car moving through rain. More raindrops hit the front window than the rear one as the car moves forward. In our terms, more meteors appear when we are facing in the direction of motion – towards the apex – than at other times. In the early part of the night, the apex is well below the horizon. Then, meteoroids have to catch up with the trailing edge of the Earth to be seen, the “rear window” of our car in the rain. At midnight, the apex rises, and as it gets higher in the sky, so the “forwards direction” (“front windscreen”) occupies more of the sky, thus improving meteor rates can be expected. The actual effect is to increase average sporadic CHRs from about 4 or 5 meteors per hour (m/h) in the early evening, to around 16 or 17 m/h in the pre-dawn hours, though there are variations in the rate over the year.

The sporadic annual rate changes occur partly because of the constantly altering elevation of the apex as it moves along the ecliptic. From the UK, the apex is highest in the early morning sky from about August to October, lowest between approximately February to April. The higher the apex, the greater the sporadic rate, in general. However, the change is partly because of an increased meteoroid concentration in the part of the Solar System the Earth occupies in the latter months of the year. In practice, mean CHRs are usually about 6-8 m/h from February to May/June, rising after that to about 15-17 m/h in November/December, though again short-term fluctuations are also seen.

Shower meteors can be observed only when their radiant is active. Shower rates vary depending on the radiant elevation and how close to a maximum the shower is. Rates will always be lower away from a shower’s peak and when the radiant is low in the sky. No two showers have identical activities, and changes are also apparent from year to year as well.

As with the sporadics, we could use an OHR to examine the actual activity, but it is more reliable to use a mean computed Zenithal Hourly Rate (ZHR) which is the corrected rate an ideal observer would see under perfect skies with the radiant directly overhead. This is then comparable to the sporadic CHR and, as with that rate, shower OHRs will normally be lower than the ZHR.

Aims of the Section

The general function of the Section is to make good astronomers, and in particular meteor observers, of its members. By following the instructions and carrying out observations as detailed below, you will be doing some original work, which is the best way to learn. Never be afraid to ask questions if you are not certain what to do or want more information on any meteor-related topics, and do not worry about your inexperience or making mistakes when you start out. Every experienced observer made – and still occasionally makes – mistakes; that is how they gained their experience. Regular observing is how to become a better meteor watcher! Don’t just focus your efforts on the night of maximum. Do make an effort to observe on other nights. Obviously, light pollution reduces the observed rates from many parts of the UK nowadays, but there are still many nights during meteor showers when useful work can be carried out.

All observers, whether veteran or novice, are to some extent unreliable, as two people at the same place seeing the same meteor can (and will) give totally conflicting reports of the event, while still believing their own is the correct version! Discussing the meteor between members of an observing group is definitely not recommended. The important thing is to try to record the details that actually happened, and this is not always the same as what you thought you saw. Remember, practice makes perfect!

Meteor Showers

A separate listing of meteor showers known to be active during the year can be found here. Since some meteor shower parameters change from year to year, it is updated annually to enable us to keep it as up-to-date as possible. Brief notes on each shower are also given there, and further information can be found regularly in various SPA sources, including the Forum, as well as the printed Popular Astronomy.

Making Meteor Observations

There are a number of ways of observing meteors. Radio sets can be used, and some amateurs follow meteor activity 24 hours a day with suitable radio receivers. Although this is a technically-challenging field, a separate Radio Meteor Observing Information page is available for those considering trying. Automated CCD video systems are used by some amateurs to collect high-quality data on meteor activity every clearer night in places. Telescopes and binoculars can be pressed into service, and enable meteors too faint for the unaided eye to be observed, while DSLR and video cameras can image the brighter meteors. Some notes on both these methods are given later. What makes meteor observing virtually unique in modern astronomy, however, is that valuable data can still be collected using only the naked-eye.

Historically, meteor paths were plotted onto special gnomonic star charts – the only sort of sky map on which meteors can be drawn as straight lines (the star charts in, for instance, “Norton’s 2000.0” were not suitable for this). This method did require experience and regular practice to achieve the necessary accuracy. Detailed studies of minor showers were carried out in this way. However, recent advances in the video monitoring of meteors have largely made this method redundant.

Visual Meteor Observing

The term Visual Meteor Observing is generally used to refer to observing with the unaided eye.

In a visual meteor watch, the watcher goes outside on a clear, dark night well away from full Moon, and observes the sky for as long as possible, writing down, or speaking into an electronic voice recorder, the details of individual meteors as they are seen. About five nights to either side of full Moon each month are rendered more or less useless for meteor work by its presence. The actual nights lost vary slightly during the year.

Here is a detailed Guide to Visual Meteor Observing

Fireballs

Fireball brightness estimates are never easy, as convenient comparison objects are rarely close by. Above magnitude -5 (Venus), there is only the Moon as a guide, and converting from an area of light (the Moon) to a point (the meteor) brings its own problems, so the brilliance of such meteors is usually more guess than estimate.

The commonest case is to overstate the fireball’s brightness, so it can be useful to be critical of your own estimates and to keep your bright-source calibration up to scratch by checking the apparent and actual magnitude of Venus regularly, as well as the various lunar phases. Shadows may be cast by meteors as “faint” as magnitude -7 or -8 at times, though this fact is often used to suggest a greater brightness. If in doubt, use a possible range of values, e.g. -7 to -9, rather than a specific figure.

Anyone spotting a fireball, whether on a meteor watch or not, should report it to the Section as soon as practical. See our Fireball Reporting webpage for more advice.

Imaging Meteors

Images provide far more accurate meteor positions than visual, or even telescopic, plotting. These images can then be used to help determine shower radiants, as well as providing a permanent record of some meteors.

In the past, meteor imaging was carried out using photographic film. Nowadays, however, astro-photographers use digital cameras.

There is no way of predicting exactly where a meteor will appear, so the main requirement is to image a wide area of sky continuously in the hope that a meteor will appear in the frame. As in other types of meteor observing, meteors close to the radiant will be difficult to recognise due to their short paths against the star background. The general recommendation is to set up your camera to point about 50° above the horizon and 30° to 40° from any active shower radiant.

Bill Ward has written a very useful guide on Imaging Meteors with DSLR cameras

When reporting your observations, please give details of all meteors imaged, as well as the start and end times and the total duration for the exposures, and remember also to state the camera’s type, lens-size and settings (e.g. ISO-rating). If you have provided copies of any successful images, please give details on the field of view shown, such as the angular size of the field and its centre in R.A. and Dec, or the prominent star and constellation names, along with the time of the exposure.

William Stewart has also provided a useful guide to Observing Meteors with a video camera

An information page with further tips, and covering some of the more technical aspects of meteor imaging, is available here. There is also a topic on the SPA’s Observing Forum where discussions by observers about digital cameras suitable for meteor work can be found.

Meteor Spectra

Bill Ward has also been experimenting with the capturing of spectra from meteors. This can give us clues as to the composition of the particles involved and explain why a meteor appears to be a particular colour. Capturing spectra is not easy, but is something that you might want to consider if you enjoy a technical challenge. You can find out more in this Overview of Meteor Spectra

Triangulation

If a meteor’s position against the stars is plotted by two or more observers at sites at least 40 km or so apart, it may be possible to calculate the meteor’s height above the surface by triangulation. This can also be done far more accurately if the meteor has been imaged, and if at least one image was taken by video, or another method allowing a velocity estimate, it may be possible to compute the meteoroid’s orbit before it entered the Earth’s atmosphere. With fireballs, if enough observers record accurate positional information for the meteor, it is sometimes possible to calculate an approximate atmospheric trajectory, and because these very bright meteors can drop meteorites, it may even be practical to try to recover any such fallen objects, if an atmospheric path can be worked-out. Special triangulation projects are organised by the Section from time to time, when instructions are given. Anyone interested in attempting some triangulation work at other times should contact the Director.

Radio Observing

Radio meteor observing, though more technically involved, offers the opportunity to observe continuously without gaps due to daylight or poor weather. The number of meteor echoes detected will depend on the sensitivity of the equipment involved and therefore care should be taken when interpreting radio meteor counts unless the low activity “baseline” of the system is known.

Further details can be found in this guide: Radio meteor observing

Telescopic Observing

Telescopic observing, using a small telescope or a pair of binoculars (7x50s or 10x50s) can produce results that are of much greater accuracy than those obtained by naked-eye meteor plotting and also takes the monitoring of meteors below naked-eye magnitudes.

It has suffered a decline recently due to the advent of the video observation of meteors. However, video observing incurs a financial cost, whereas telescopic observing need not if you already use the telescope or binoculars for other observational activities.

Here is a guide to Telescopic Meteor observing

Conclusion

Please always give an e-mail address that will be viable for at least a couple of weeks, as an immediate reply is rarely practical, whenever you write to any Section officials if you require a reply. If using ordinary mail, please enclose a stamped, self-addressed envelope (SAE), preferably an A5-sized one. Note that any envelope larger than A5 incurs the “Large Letter” postage rate, so please remember to provide the correct stamp on your SAE. Always try to submit your results promptly – at most within one month of their being made. Good luck and clear skies for all your observing: your results are eagerly awaited!

References

“Norton’s 2000.0: Star Atlas and Reference Handbook” (18th edition), edited by Ian Ridpath, published by Longman Scientific & Technical, Harlow, England, 1989.

“Sky Catalogue 2000.0: Volume 1 – Stars to Magnitude 8.0”, edited by Alan Hirshfeld and Roger W. Sinnott, published by Cambridge University Press, Cambridge, England, 1982.

By Alastair McBeath, Tony Markham, Shelagh Godwin, David Entwistle & Robin Scagell

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