Ephemeris Time (ET) and Terrestrial Dynamic Time (TDT) are both stable time standards devised to replace variable solar time. They are extremely precise methods of measuring time. The difference is that Terrestrial Dynamic Time is based on atomic time, while Ephemeris Time is based on the movement of celestial bodies. Once atomic time was developed in 1955, Ephemeris Time became obsolete. TDT is essentially the replacement for ET.
A day is as long as it takes for the earth to make one complete rotation on its axis. We could measure time in this way and discover how long a second lasts by dividing the number of seconds in a day by the length of the day – if the earth’s rotation was constant. It isn’t. Because of the pull of the moon on the earth’s waters, the earth is spinning at a slower rate each day. This is called tidal deacceleration. The change is small, and for most time keeping matters, inconsequential. Civil time is still based on mean solar time. But in some cases, such as for making astronomical calculations, a stable standard of time is necessary.
Ephemeris Time was developed for just this reason. In 1950 the conference on Fundamental Constants in Astronomy in Paris, experts suggested basing the time standard not on the variable rotation of the earth on its own axis, but the movement of the earth around the sun. This method was introduced two years later by the International Astronomical Union. A second became defined as a subdivision of one year, or to be precise, 1/31, 556, 925.9747 of the year 1900, beginning on January 1st, on the 12th hour. One of the problems with ET is that it is based on a particular year in the past – it can’t be measured again. Also, although ET worked well for astronomical reasons, navigators still depended on solar mean time. Astronomers used the Ephemeris Time standard up until 1979, but it inevitably became a less viable alternative to solar time with the introduction of atomic clocks.
Atomic time is based on the frequency of atomic oscillations. It is a near perfect way of keeping time, precise down to microseconds every year. The official atomic time (TAI) is taken from the average time of 300 atomic clocks in 50 countries. Terrestrial Dynamic Time is based on atomic time. It is atomic time plus a constant of 32.184 seconds. This constant is the amount of time necessary to continue the Ephemeris time count for the instant of transition, which was at the beginning of January 1, 1977. So although ET was eventually phased out, it was so accurate in itself that it was used to calibrate atomic time. Terrestrial Dynamic Time became the official time on the surface of the earth in 1984.