A NOTE ON COMPUTING ASTRONOMICAL PHENOMENA
01/05/2019

For amateur astronomers, almost everything you see in the sky is only loosely tied to the clock.  For example, when an object is rising or setting its position in the sky is altered by atmospheric refraction, and the observation, unless you are at sea, is also heavily influenced by the local topography. The timing of a transit can, with professional equipment, be timed to about a millisecond, but in general one-second timing, or tenth-of-a-second, might be the best you could expect.

The timing of greatest elongation from the sun, closest approach, etc. are phenomena in which the measurement in question is changing very slowly, so a precise time for the event has little meaning.  Thus, a time of "21:39:01 on 04/07/2017 UTC" for "Jupiter at opposition" really tells you only that Jupiter will be an all-night object on the night of April 7th in 2017.  Traditionally, this time is only listed to the nearest hour, but varying the round off according to the phenomenon would require extra logic for little benefit.

However, in the case of solar eclipses (total or annular), solar transits of Mercury and Venus, and lunar occultations of stars, split second accuracy is possible even for amateurs - provided you have an accurate clock available, and very accurate coordinates for your observing location.  For solar eclipses and occultations analysis of your data (or predictions) the major difficulty is the need for extreme accuracy in the calculated lunar ephemeris, and such parameters as the diameters of the moon and the earth.  Also note that high accuracy for these phenomena requires accurate data for DUTC (= TT - UTC) and DUT1 (= TT - UT1), because basically the calculations are accurate in terms of TT, but the difference between civil time based on TT-DUTC and the rotation of the Earth on its axis depends on knowing UT1 (UT1 = TT - DUT1).  Both DUTC and DUT1, for future dates, are somewhat uncertain.  For more information on this, click here.

The listings provided on this site should be accurate enough to allow planning of observing sessions by most amateurs in astronomy.

George Gladfelter
Rapid City, South Dakota