[Illustration: 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. [Illustration: Diagram (not drawn to scale) showing plotted compass directions to the last visible point on a fireball path. (The point denoted by L in next diagram.) Black dots represent positions of various observers. Each arrowed line is directed toward the last visible point as it was estimated by the individual observer. The oval area, which includes points of intersection of all observed lines of sight taken in pairs, marks out region in which meteorites have probably fallen.]