What is a day?

When does the day start? The ancient Greeks said the day began at sunrise, and ended at the next sunrise. The Babylonians held that the day began and ended at sunset, while up until 1925,

astronomers worked on a “day” which began and ended at noon, and so did the Royal Navy in the days of Captain Cook. Now the astronomers, like us, and the ancient Egyptians, have a day which commences at midnight. Islamic tradition has a day which begins at sunset.

By definition, noon is when the Sun is at its highest point in the sky, so it is always noon somewhere in the world, with a noon zone sweeping along through a degree of longitude every four minutes. Logically, you should be setting your clock forward or back by a minute for each 15-20 kilometres that you go east or west!

This would be far too confusing, and to make life easier, we have split the world into time zones, usually (but not always) 15 degrees across, where everybody keeps the same “official time”. If you are trying to set up a very accurate sundial, you need to make allowance for your position east or west of the true time in your zone.

Some points to ponder: Clockwise is a word used to describe the direction of the shadow of a northern hemisphere sundial. What way does the shadow travel in the southern hemisphere?

If you stop and think about it for a moment, you may be able to deduce where the word “dial” comes from, especially if you know anything about the Latin word dies. If you lack this knowledge, look up “dial” in a good dictionary, and find out where it comes from. From this knowledge, can you say what the most appropriate use of the word “dial” is?

We probably had the idea of the two ways, even before clocks. Widdershins is an old word meaning counter-clockwise. The equally old word which means “clockwise” is deasil.

Somewhere along the way, something happened to humans that made them start using art, that made them start communicating with each other, and generally showing signs of being human, rather than hominid. Could it have been the discovery of time which caused these changes?

It is probably time to look at the things we first used for time-keeping and calendars, the stuff that is out there, beyond our atmosphere, the stuff that even a century ago, people knew was forever unreachable — and rather hard to see in any case.

The barman says, “We don’t serve time travellers in here.”

A time traveller walks into a bar.