32. Lots of Times

Standard everyday time is based on the rotation of the earth. This Universal Time (UT) is unsuitable for astronomical calculations, because the earth's spin is slowing down in an irregular manner.

Astronomers use a uniform time called Ephemeris Time (ET) in their calculations. Delta-T = ET-UT is a measure of the difference between the two ways of tracking time.

According to historians, the mean solar day in Diary times was about 1/20th of a second shorter than today. The minuscule difference added up over the nearly million days since 500 BCE and now amounts to several hours. This accumulated clock difference equals delta-T.