by Phil Reed and Ramesh Pooran
While almost everyone can enjoy a meteor shower, only a few understand the dynamics behind these pebbly interplanetary interlopers. A lot of people will know that there's a meteor shower sometime in August each year, but not many can tell you where or when to look. When you get right down to it, even the experts don't really know. All they can do is estimate a time and location based on observations from previous years. This is where a carefully planned meteor watch can yield usable scientific data.
The dynamic behaviour of meteor showers is a direct consequence of their genesis as cometary debris. As a comet nears the Sun, the ices (water ice, frozen CO, methane, etc.) on its surface evaporate, liberating any rocky particles that were embedded therein. These little chunks of rock continue in much the same orbit as the comet, but now they are subject to a variety of interactions. They can collide with each other; they can be affected by radiation pressure from the sun; or they can be gravitationally influenced by the planets. The comet itself may be rotating and its ices may evaporate in fits and starts, further adding to the random motions of the now liberated particles. As a result, these particles, ranging in size from dust motes to fist-sized rocks, spread out around the parent comet. As time goes by it is easy to imagine that the entire orbit of the comet becomes populated with these comet crumbs. If the orbit of the comet is such that it intersects the orbit of the Earth, then we Earthlings are treated to a meteor shower each time our planet ploughs through the stream of debris. Incidentally, the particles are called meteoroids. When these meteoroids burn up in the Earth's atmosphere they are called meteors, and any that survive the fiery descent are called meteorites. Not all meteorites (and hence meteors and meteoroids) are of cometary origin. Some come from the asteroid belt, a few originate from Mars or other planets and moons, some may be from interstellar space. However, there is little scientific doubt that our regular meteor showers are spawned from comets.
Because the Earth crosses any meteoroid stream at about the same time each year, the resulting meteors appear to originate at roughly the same point in the sky. The name of the meteor shower is often derived from its apparent point of origin. Thus our beloved Perseids seem to emanate from an area within the constellation Perseus. The Geminids originate in Gemini, Orionids from Orion, and so forth. There are a few exceptions that a good astronomy text book will explain in mind-numbing detail. Meteors that are not associated with any shower are lumped under the term "sporadic."
The streaks of light that make up a meteor shower are not all the same. Indeed, each meteor shower has its own characteristics. Some showers, such as the Perseids, consist of fast bright meteors which leave an incandescent trail; others may have a lot of faint meteors. The year to year variation can be quite significant. Some meteor showers may routinely have peak rates of a couple of dozen per hour for many years in succession and then suddenly explode with rates of several hundred (or thousand!) per hour, only to return to low rates the following year. With this degree of variation, prediction of meteor shower behaviour was for a long time more of an art than a science. However as more and more data was collected, professional astronomers began to paint a more coherent picture of meteor shower behaviour.
The concept of a "meteor watch" as a scientific tool was driven by the desire for more and better data about the meteoroid stream. Reduced to its simplest, a meteor watch consists of a group of people who record the number of meteors that they see during a given time interval. In reality, to get usable data you have to record the time at which the meteor was seen, its magnitude, its direction of travel and whether or not it left a glowing trail. The time information is important because it will yield information about the particle density of the meteoroid stream, i.e. a poorly populated stream will provide only a few meteors per hour. The meteor magnitude is principally related to the meteoroid size: marble-sized particles give brighter meteors than sand-grain sized ones. The direction of travel will serve to identify the apparent source in the sky and ascertain if it is a "sporadic" meteor. Another important factor is the limiting magnitude of the observing site. Seeing fainter stars imply that it is possible to see fainter meteors which in turn gives a better picture of the smaller components of the meteoroid stream. Collecting and interpreting all this data will lead to a description of the current meteoroid stream. Comparing this data with that collected from previous years may allow an educated guess about next year's expected meteor shower performance.
This was the first year we had a meteor watch at Starfest. Even so, we had a very good turnout. About a dozen people showed up with lawn chairs and endured the extremely cool night to participate in this event. The observers have to be arranged so that they can each watch overlapping pieces of the sky and collectively cover the entire sky. This was accomplished by forming the lawn chairs into a rough circle so that the sky-watchers could sit with their feet pointed towards the centre This allowed any patch of sky to be simultaneously seen by at least two people. Each observer was asked to yell out "Time!" and provide a short description when any meteor was seen. A tape recorder was used to collect verbal information about each meteor (magnitude, trail, direction) and a short-wave radio tuned to WWV provided the time signal. In a one hour period the group collectively observed two dozen Perseids, several of which were brighter than first magnitude, as well as a few sporadics. The diagram above shows the time distribution of the Perseids during the observation period. Some of the magnitudes had to be estimated because the tape recorder did not capture them very well. However, the data clearly shows a twenty minute period when only two Perseids were seen. Also, even though the sample is small, there's just a hint that the Perseids are a little brighter and more numerous after local midnight.