Page:Popular Science Monthly Volume 4.djvu/226

214 time for her disk to move from first contact (when her disk just touches the disk of the sun exteriorly) to second contact (when her disk is tangent interiorly to the sun's), and during this time the appearance of the two disks will be as in the figure:



This figure shows Venus coming on to the sun's disk, and it shows the two cusps at a and b. It will be easily seen that, if we know the length of the line a b, and the time at which it has this length, we can calculate the time of contact from these data. So that a number of measures of the cusps is the same as a number of first contacts. The reverse phenomenon occurs when Venus passes off the sun's disk.

To measure these distances, the equatorials must be provided with filar micrometers, i. e., with a contrivance by means of which two spider-lines in the focus of the telescope may be moved toward or away from each other. One of these lines is to be placed at a, and the other at b; the time is to be noted, and the number of turns and parts of a turn of the screw which moves the lines is to be noted from the head of the screw, which is finely divided.

3. The photographic method. This consists in photographing the planet Venus on the disk of the sun, and noting the time of each photograph. The negatives are carefully preserved, and are measured subsequently by a fine measuring engine. It will be seen that this method is like the preceding, except that the measuring may be done