The measurement of time is a fascinating subject, Superficially simple and commonplace, it develops more and more complexity the more you look into it. In learning what I needed to know to design a sundial, I found myself delving into the subject and often frustrated that the information I needed was spread out over multiple sources, and not always consistent between them. So here is my attempt to summarise what I think I know.
UTC &c
There are several different definitions which capture the fact that when high accuracy is needed, none of the earth’s movements is completely regular. In particular:
- IAT is international atomic time, and defined in terms of atomic clocks. The fundamental unit is the second.
- UT1 is earth-based time. The day is defined first, as the mean day, and the second is a precise fraction thereof.
- UTC defines the day as the UT1 day, and the second as the IAT second, and uses leap seconds to keep the two in sync.
Equation of Time
The offset for the published equation of time is chosen so that over the course of the year it averages to 0. For some reason, this fact is hard to find. To put it another way, the average difference between mean and apparent time is 0 over the course of a year.
