Q1.
But how are these periodic events used for keeping time?
Answer
By counting them. Any event that repeats after a fixed interval can be used as a unit, and measuring time is simply counting how many of those intervals have gone by. The three cycles in this chapter give the three units we still use.
| Natural cycle | What is actually moving | Length | Unit of time it defines |
|---|---|---|---|
| Sun returns to its highest point in the sky | Earth rotating on its axis | 24 hours | The (mean solar) day |
| Moon completes one cycle of phases | Moon revolving round the Earth | About 29.5 days | The month |
| Seasons return to where they started | Earth revolving round the Sun | About 365¼ days | The year |
Why it happens: A calendar is nothing but a scheme for packing the smaller counts neatly inside the larger ones. The trouble — and the reason different kinds of calendars exist at all — is that the three cycles are not whole multiples of one another. Twelve lunar months are 354 days, not 365, and the year is not a whole number of days either. Every calendar in the chapter is one particular way of handling those two awkward leftovers.