and others in France, led to a reasonably accurate estimate of the distance of Mars and hence of that of the sun. Mars was at the time in opposition (chapter ii., § 43), so
Fig. 68.—Mars in opposition.
that it was nearer to the earth than at other times (as shewn in fig. 68), and therefore favourably situated for such observations. The principle of the method is extremely simple and substantially identical with that long used in the case of the moon (chapter ii., § 49). One observer is, say, at Paris (p, in fig. 69), and observes the direction in which Mars appears, i.e. the direction of the line p m; the other at Cayenne (c) observes similarly the direction of the line c m. The line c p, joining Paris and Cayenne, is known geographically; the shape of the triangle c p m and
the length of one of its sides being thus known, the lengths of the other sides are readily calculated.
The result of an investigation of this sort is often most conveniently expressed by means of a certain angle, from
