Now, let us move into the third dimension, since a fireball’s path through the atmosphere lies in space, not in the “flat” plane of the earth’s surface. Returning to our baseball diamond, let us suppose that a helicopter with an enterprising photographer aboard hovers over the centerfield bleachers so that he can take pictures of the record crowd. While the umpire is dusting off home plate, the two runners on first and third simultaneously sneak a look to see what the helicopter is doing. Their lines of sight now intersect at the helicopter and fix its position in space.
Similarly, the location of a fireball path in space is determined by the fixing of certain points on the luminous streak seen in the sky. Instead of using only two intersecting lines of sight (those of the runners on first and third in our analogy), scientists investigating a meteorite fall try to collect as many different lines of sight as possible from people in the region above which the fireball streaked. The more commonly determined points are those of the fireball’s appearance and disappearance and those where “explosions” took place. These points are generally located by use of the method we have described in some detail above, the so-called intersecting-lines-of-sight method.
The most important point on a fireball path is the point of disappearance. The most valuable single piece of information you can supply about a meteorite fall is as accurate an answer as possible to the question: In what compass direction were you looking when you last saw the fireball? This question has often been twisted around in newspaper and radio accounts into the meaningless question: In what direction was the fireball going when you saw it?
One person cannot give the answer to the second question because from a single station it is impossible to determine the true direction of motion of an object seen in the sky. One person can report only an apparent direction of motion, which is of little or no value in locating the last point on the luminous path, generally referred to as the “end-point.” Therefore, though you cannot by yourself determine the actual direction in which a fireball is moving, you can report the direction in which you were looking when you last saw the fireball, that is, due south, southwest, northeast, etc.
O is an observer squinting along the top of a ping-pong table. A ping-pong ball rolls along the top of the table from B (beginning) to E (end). To the observer at O, however, the ball would appear to start at B and end at E if it rolled along any one of the dashed lines leading from OB to OE. By means of a similar space-figure, it can be shown that a single observer at O cannot determine the true direction of motion of a luminous object in the sky, like a meteor.
Scientists are eager to obtain reliable reports on the compass direction to the fireball’s point of disappearance from as many widely separated eyewitnesses as possible. They then can plot the individual lines of sight on a good map, marking exactly where these lines intersect. In this way, the investigators can make reasonably accurate fixes of the position of the point on the earth’s surface that is situated directly below the end-point of the fireball path, as this end-point was seen in the sky by each pair of eyewitnesses.
Instead of using the ordinary compass direction to a fireball’s point of disappearance, you may prefer, as do astronomers, to use the azimuth. What we have been calling a “compass direction” is one that is expressed in terms of the cardinal points: north, south, east, west. An azimuth is a direction stated in degrees. Rough azimuths can be taken with a compass, but for accurate work, a graduated circle, like that on a transit or theodolite, must be used. Astronomical azimuths begin at the south point and continue clockwise full circle to 360°. For example, the lines of sight in the diagram, [p. 87], could very well have been given as astronomical azimuths. And, in the diagram, [p. 91], the line of sight C₁ could have had the precise designation 118° and C₂ that of 222°.
Every fix serves to guide field parties to areas that are to be carefully searched for fallen meteorites. Extra-thorough searches are made if the people living in a particular area reported that they heard meteorite fragments hissing and whining on their way to earth or heard the thumps of their impacts on the ground.
You will notice that so far we have been treating our problem as a two-dimensional one. We have been working with directions only and have plotted out direction indicators on a map representing the plane of the earth’s surface. Now, as we did in our baseball analogy, let us move into the third dimension.