From earliest times, observed astronomical cycles were used to mark the passage of time. Calendars were established based on lunar and solar cycles, growing seasons, and religious holidays. Systems of timekeeping were developed based on the apparent motion of the Sun in the sky, but early astronomers were already able to note the variations of apparent solar time, and mean solar time was established around AD 150.
Timekeeping developed using sundials, water clocks, mechanical clocks, pendulum clocks, and chronometers. Conventions for the hour marking the beginning of the day evolved from noon to midnight and hours of different lengths evolved to those of uniform length. Time transfer techniques changed from bells, to time balls, and to the telegraph. Until the mid-20th century, the rotation of the Earth was the basis of timekeeping. At the beginning of the 20th century, official time was based on mean solar time from Newcomb’s Theory of the Sun and kept pendulum clocks set by astronomical observations. There was no international timescale, and the variation in the Earth's rotational speed was suspected, but not proven.