Dear Victor, You will have thought about this a lot more than I have. But it seems to me that all planetary phenomena will be calculated in uniform ET. So at a time ET1, which describes some astronomical event like a conjunction, delta T (ET1) describes how the orientation of the Earth differs from its value at ET1 if it had been calculated using a uniform rotation. When it comes to the Moon I imagine today the ephemeris also uses ET as the arguement. The solar system runs on uniform ET, leaving the Earth's period of axial rotation to increase, while the moon moves away from us and its orbital period arround us also increases but not as fast as the Earth's. So the moon appears to accelerate. I am no expert on all this so dont worry, I expect we are saying the same thing but in differnt ways. Best wishes Robin ---------------------------------------------------------------------------- Robin Catchpole (Dr) Phone 01223 337 553 (Cambridge) Institute of Astronomy SwitcB 01223 337 548 (Cambridge) Madingley Road CAMBRIDGE CB3 OHA mobile 077 1421 8670 (cellnet) U.K. email