Page:Astronomy for Everybody.djvu/261

Distances in the heavens may be determined by a method similar to that employed by an engineer in determining the distance of an inaccessible object—say a mountain peak. Two points, A and B, are taken as a base line from which to measure the distance of a third point, C. Setting up his instrument at A, the engineer measures the angle between B and C. Setting it up at B he measures the angle between A and C. Since the sum of the three angles of a triangle is always one hundred and eighty degrees, the angle at C is found by

subtracting the sum of the angles at A and B from that quantity. It will readily be seen that the angle at C is that subtended by the base line as it would appear if viewed by an observer at C. Such an angle is, in a general way, called a parallax. It is the difference of direction of the point C as seen from the points A and B.

It will readily be seen that, with a given base line,