Time of the season

Some hours used to be longer than others.  Really.

A correspondent of our astronomer recently posted on a social medium something like, “It’s 6:04 pm and it’s still light out!”  The point was, of course, that after what probably seemed like a harsh winter the days were getting longer again, with the sun rising earlier and setting later.  Our astronomer was tempted to remark in reply that such things have happened before, and indeed they happen every year; and that they can be predicted with a very high level of accuracy.  So there was really no reason for surprise, was there?

But he did not, because he realized that the post was not an expression of surprise.  I was rather an expression of pleasure, and (we agree) it is a sign of hope when people take pleasure in things long foreseen and actually inevitable.  He remembers the short days and long nights of winter in Edinburgh, and the seemingly endless summer afternoons in England with the sound of a cricket match in the distance.  The expression “long hours” came to mind.

But that expression comes perilously close to being meaningless.  All hours are the same length, right?  Sixty minutes of sixty seconds each, kept so by (in this country) the authority of the US Naval Observatory?  One hour cannot actually be longer than another, whatever it may feel like when taking a test or waiting for a plane.  The whole structure of timekeeping depends on it.

However, it hasn’t always been so.  If you refer to almost any astrolabe, you will find a scale that allows you to convert between equal hours (twenty-four in a day) and unequal hours (twelve from sunrise to sunset, or sunset to sunrise).  As the length of the sunlit day changes, the length of the (unequal) hour changes to match.  This actually makes a lot of sense in context.  In a society where work is done outside or at least depends on sunlight, measuring time in sunlit hours is useful.  Rather than messing around with the almanac time of sunset and running your abacus on equal hours, you could just say, “We have a third of the day left, so we’ll have to hustle to finish off this field before the bailiff takes his serfs to work elsewhere tomorrow.”  Indeed, the canonical hours of prayer (in two religions) were defined by sunrise, sunset and an even division of time between.

Our navigator suspects that the shift to equal hours day and night began with long-distance sailing.  To work out even roughly where you are at sea, you need an estimate of how fast you’re going, and that’s hard enough to do in the normal course of things without your units of time growing and shrinking on you.  It may have been common to shorten sail at night in unfamiliar waters, but that would cut your profit on a trading voyage, and it would be difficult (and pointless) to try to match the change in unequal hours.

So you probably have sailors to thank for the fact that the most trouble you have with your clock is shifting to and from Daylight Savings Time.