Page:Astronomy for Everybody.djvu/262

the greater the distance of the object the less will be its parallax. At a sufficiently great distance the latter will be so small that the observer cannot get any evidence of it. To all appearance the lines B C and A C will then have the same direction. The distance at which the parallax cannot be made out depends, of course, on the accuracy of the measurement, and the length of the base line.

The moon being the nearest of all the heavenly bodies has the largest parallax. Its distance can therefore be determined with the greatest precision by measurement. Even Ptolemy, who lived only one or two centuries after Christ, was able to make an approximate measure of the distance of the moon. But the parallax of a planet is so small that it can be determined only with the most refined instruments.

The ends of the base line used in the determination may be any two points on the earth's surface—say the observatories of Greenwich and the Cape of Good Hope. In the case of the transits of Venus, which we have already described, there were a number of different stations at various points on the earth's surface, from which the direction of Venus at the beginning and end of its transit could be inferred. This method of determining distances is called triangulation.

The idea of a triangulation, as thus set forth, gives an understanding only of the general principle involved in the problem. One can readily see that it would be out of the question for two observers in distant parts of the earth to get the exact direction of a planet at the same