The time-scale that was used in calculating orbits within the Solar System from 1960 to 1984. Its fundamental unit was the ephemeris second which was defined by the statement that the tropical year at the epoch 1900.0 was precisely 31 556 925.9747 ephemeris seconds. The only accurate way to determine Ephemeris Time by observation was to measure the position of the Moon against the stars, for which a device known as the Markowitz Moon Camera was invented. Ephemeris Time was in many ways inconvenient, and in 1984 it was replaced by Terrestrial Dynamical Time (now known as Terrestrial Time) which has the SI second as its fundamental unit.