Use of the gravity pendulum as a timepiece for longitude determinations

Longitude determinations of a precise nature are made by measuring the difference in longitude between a point of unknown position and a point whose position has been very accurately determined. The longitude difference can as well be expressed as a time difference and is, in fact, so measured, and it is evident that the accuracy with which the time difference is known is the sole factor determining the accuracy of the results of the longitude observations. The modern method of measuring differences in longitude is to compare time as obtained by local star observations with a radio time‐signal sent at a known time from a point of known longitude.