ephemeris time

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A highly accurate astronomical system for the measurement of time based on the period of Earth's orbit, but in practice relying on lunar observations and an accurate lunar ephemeris.

Wiktionary

1. n. astronomy A former standard astronomical time scale intended to overcome the drawbacks of irregularly fluctuating mean solar time, superseded in the 1970s.
2. n. astronomy A modern relativistic-coordinate time scale.

WordNet 3.0

1. n. (astronomy) a measure of time defined by Earth's orbital motion; terrestrial time is mean solar time corrected for the irregularities of the Earth's motions