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Detailed Fireball Reporting Guide

Contents

Essential items to record

A fireball sighting should be recorded immediately after the event. This is very important, as you may easily forget or accidentally exaggerate some details if you delay. The three essential points to give in your report are:

  1. Exactly where you were (give the name of the nearest town or large village and county if in Britain, or your geographic latitude and longitude if elsewhere in the world, as well as the place and country name);
  2. The date and timing of the event (please be sure to state clearly whether this was in Universal Time, UT, which is the same as Greenwich Mean Time, or in local clock time); and
  3. Where the fireball started and ended in the sky, as accurately as possible, or where the first and last points you could see of the trail were if you did not see the whole flight.

Estimating sky-positions

In order to try to reconstruct accurately what you saw, so it can be compared with what other witnesses elsewhere spotted, and thus attempt to triangulate to its atmospheric trajectory, it is absolutely vital you try to describe where the meteor was in the sky as clearly as you can. The key elements are to note the positions for the start and end of its visible trail, or the first and last points of that trail if you did not see the whole thing (which is quite common).

If the fireball was seen at night with plenty of stars visible, and you are familiar with the astronomical convention of using Right Ascension, RA, and Declination, Dec, to describe sky-positions, you can give these points in RA and Dec.

If the fireball was seen in daylight, or twilight too strong to see sufficient stars, or if you are not familiar with estimating RA and Dec positions, please use altazimuth positions instead.

  • Altazimuth system: The altazimuth system is a means of giving angular positions in the sky based on the true horizontal for where you were at the time. Any point in the sky can be determined by two quantities, the altitude or elevation in degrees measured vertically up from the horizontal, and the azimuth, literally "directions", in degrees measured around the sky parallel to the horizontal. Altitude starts at 0° at the horizontal and runs up to the zenith, or overhead point, at 90° elevation. Azimuth is measured from true north eastwards (that is, clockwise), thus 000° is due north, 090° is due east, 180° is due south, 270° is due west, and so forth. It is useful to give altitude values as two-digit numbers, and azimuth ones as three-digits to help you avoid confusion.

To help you in making these angular measures, the distance across the clenched knuckles of your hand held at arm's length from your eye gives an angle of about 10°. The distance between the outer edges of your thumb and fourth finger of your outspread hand, again held at arm's length, forms an angle of about 22°. A single fingertip at arm's length makes an angle of about 1°. You can test this smallest measure quite easily against the diameter of the full Moon, which is about half a degree across. If you measure the azimuth positions using a magnetic compass, please be sure to state this, as there is a discrepancy between true and magnetic north which sometimes needs to be allowed for. If you are concerned you may have given the positions incorrectly, you can easily allow a rough confirmation by also stating in your description the approximate compass directions, such as 'south-southeast', 'northwest', etc., as well as the azimuth.

Other items to note

The following elements are listed in a crude descending order of importance and/or their relatively lower chances of occurring with any given fireball.

  • Apparent speed: If you are reasonably familiar with meteor apparent velocities, you can use the usual 0 to 5 meteor-speed scale from "Observing Meteors", where 0 = stationary (the fireball appeared as a point of light, as it was heading directly towards you through the atmosphere; also called a point-source meteor), 1 = very slow, 2 = slow, 3 = medium speed, 4 = fast and 5 = very fast. The scale gets easier to use the more meteors you have seen. As compared to objects like distant aircraft or artificial satellites, even a very slow meteor can seem remarkably quick! If you have little or no experience of seeing meteors, you may find it easier to estimate how long the fireball was visible for in partial or complete seconds. Very few fireballs last more than 5 or 10 seconds, and only man-made re-entry fireballs are likely to last for several tens of seconds up to a couple of minutes. If the object was visible for longer than these times, it may not have been a fireball. Some objects which may be mistaken for fireballs are discussed below under the "Fireball or not?" section.
  • Trains: Some meteors, especially the faster and brighter ones, may leave a glowing ionization train along their trajectory after they have disappeared. If so, estimate how long this took to fade from view in seconds. Rarely, the train may last several minutes, when sketches could be made to show how it changed over time, or images taken. Very rarely, typically seen only with deeply-penetrating meteors, a dust or smoke train may be left along the object's flight path. The duration, appearance and colour should be noted if so.
  • Sounds: Noises may occur associated with some brighter fireballs. Sometimes these occur simultaneously with the meteor's flight, and are typically described as hissing, whooshing or crackling, though others have been reported. Although long dismissed as illusory, a tiny number of these electrophonic sounds have been recorded, so some are genuine. They may be due to very low frequency radio waves being detected audibly by some unknown mechanism, and are extremely rare. It is important to rule out as far as possible any terrestrial noises that happened by-chance at the same time as the meteor appeared, such as the wind in leaves or overhead wires, or even the sound of your hair brushing against your collar, but if you are convinced the sound probably did not have such a mundane cause, you should also send details from your report to the Global Electrophonic Fireball Survey. More common, but still very rare, are noises heard tens of seconds to a few minutes after the fireball has ended, usually rumblings or bangs. These are due to acoustic shock waves passing through the atmosphere, "sonic booms", often caused by severe detonation events in the fireball's flight. Again, terrestrial causes need to be ruled out where possible. In all cases, note down what sounds you heard, and what the approximate delay was between your seeing the meteor and hearing the sounds.
  • Fragmentation: Give details of any fragmentation the fireball showed, which could include a breakup into several smaller meteors (try to state their number if possible), or sparkling material seen along the track while the object was in flight, possibly due to molten material spraying off a rotating main body.
  • Colours: Describe any colours seen using only pure hues, that is: red, orange, yellow, green, blue, violet, white. So state "blue-green" not "turquoise", for example. The human eye is not sensitive enough to usefully tell what chemical components may have produced which colours, and of the spectroscopically-identified meteoric elements, unfortunately far too many are capable of generating common fireball colours, such as blue, green and yellow. Some of the colours result from atmospheric components anyway.
  • Magnitude: Try to give an estimate of how bright the event was. This is probably the most difficult thing to say with precision. In Observing Meteors there are some notes on estimating fireball brightnesses and a list of comparison objects which may be of help. Details from the comparison list are as follows. The planet Jupiter at its brightest reaches magnitude -2.9 (its opposition magnitude range is -2.3 to -2.9), thus any meteor as bright as Jupiter or brighter will be a fireball. The planet Venus at its most brilliant reaches about magnitude -4.8, but after that we have only the Moon as a guide (you are not likely to see a fireball anywhere close to that of the Sun). The 4 or 24 day old crescent Moon is about magnitude -8; first or last quarter Moon is around magnitude -10; the 11 or 18 day gibbous Moon is roughly magnitude -11/-12; and full Moon is magnitude -13. However, bear in mind that the Moon's light is spread over its disk, so a mag -13 fireball would actually be somewhat brighter than the Full Moon. Over-estimating a fireball's brightness is very common, so if in doubt, use a range of possible value, such as -7 to -9, without guessing at a single number.

Reporting your sighting

If you complete the SPA's detailed report form, please also report your sighting to the international fireball database via this form: International fireball report form