Re: Units of Measurement
"Atomic time" is not based on "Ephemeris Time". "Atomic time" (TAI) is the average of a bunch of atomic clocks. "Ephemeris Time", well these day TT(TAI), is the independent variable used to calculate ephemeris. If your clock advances 86,400 second of atomic time then you add 86,400 to the coordinate used in your ephemeris and consult your model to determine where the sun, moon and stars will be.
The leap second changes neither "Ephemeris Time" nor "atomic time", but adds 1 SI second to UTC (wall clock time) so the difference between the wall clocks and atomic clocks (UTC-TAI) increases from 35s to 36s which ensures the the (mean) sun is overhead at midday.