Subsequently, to obtain a time scale, some periodic phenomenon which has repetitions occurring at a uniform rate that may be subdivided and counted must be used. Before the 20th century it was assumed as self-evident that a single, universal, uniform time scale existed. Albert Einstein , however, early in the 20th century, recognized that because of the universal constancy of the speed of light the measurement of time depends on the motion of the observer. Thus, in the modern view, time is no longer absolute, but dependent on the relative motion of observers making the time measurements. Time and space are different aspects of this underlying four-dimensional continuum.
Frequently time is described as a fourth dimension.
In order to specify a date, using the apparent motion of the Sun as a time scale, days must be counted from some reference date. In addition, a clock is used to measure fractions of a day. Time derived from the apparent position of the Sun in the sky is called apparent solar time. Because of the eccentricity of the Earth's orbit around the Sun and the inclination of the Earth's rotation axis to the orbital plane, apparent solar time is not a uniform time scale. These effects can, however, be calculated and corrections applied to obtain a more uniform time scale called mean solar time.