The time as measured by any observer from a clock at their location that is stationary with respect to them. In the theory of relativity, time is said to be proper to each observer. This relativity of time is an essential feature of the special theory of relativity, and makes a moving clock appear to run slow (see time dilation). Each observer will record the clock as ticking at a different rate, because each is comparing it with their own proper time. The time recorded on the clock face, which is something all observers can agree upon, is the clock’s own proper time. In the general theory of relativity a gravitational field also makes a clock appear to run slow. In both general and special relativity the proper time interval between two neighbouring events can be defined as the time measured by an observer present at both of them. Terrestrial Time, the time-scale used in The Astronomical Almanac for geocentric predictions, is the proper time for an observer on the Earth’s surface.