Sunrise to sunset: the most basic of time references. Light is essential to human activity, and so for early man hunting and gathering were daytime pursuits. For very basic timekeeping, little more was required; when the sun rose it was time to eat and begin activity, when it set again it was time to sleep.
But as man became involved in more and more complex tasks it became apparent that a system with shorter intervals was required. In ancient civilizations, particularly the Egyptians, counting systems used twelve as a base as opposed to the modern ten. This is due to the presence of twelve finger bones on each hand (three bones on four fingers), and the ancient world’s preference for twelve dictated all numbering – twelve objects became known as a dozen, twelve zodiacal signs were identified and the day was split into twelve sections.
Wait a minute, you say, there are twenty-four hours in a day! Well, these early civilizations realized that daylight and darkness were, on average, roughly equal. It made sense then to have twelve daylight units and another twelve darkness ones. Hence, a complete rotation of the Earth had been defined as twenty-four units.
Over the years various names were given to this 1/24th of a day. The Ancient Greek word ‘hora’ roughly translates as ‘time of day’, but the English name ‘hour’, was not recorded until the 13th century.
Given that one complete rotation of the Earth – and the subsequent light-dark cycle that results – is what makes a day so useful it is unsurprising that it was the unit from which smaller units of time were derived. It was decided that since the sun reached its highest point halfway through the daylight section, it would make sense to call this the mid-point, or twelve hours into the day. It then followed that at zero or twenty-four hours (which are of course the same) the day ended and the next one began.
As accuracy became ever more important, the hour itself was split up into smaller sections. The Babylonians used base sixty numbering, which although it sounds rather complicated is actually very useful since sixty is divisible by 2, 3, 4, 5, 6 and 10. As a result, their hours were divided into sixty minutes, and this useful time system soon spread to other civilizations.
Seconds were developed in a similar manner, and got their name from the division used: An hour was divided first by sixty to get minutes, and then by sixty a second time to get… well, seconds. In some languages, such as Polish, a sixtieth of a second is still called a ‘third’ for this reason.
In modern times, highly accurate measurements of Earth’s rotation have led scientists to calculate that the second is equal to “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom” (according to International Standard ISO-31). Now, I don’t know what that means exactly either, but I do know that it is a very accurate way of measuring time based on counting radiation, and ultimately was still defined in comparison to the rotation of our planet.
Of course, larger units of time have been devised – a year is the time it takes for the Earth to orbit the sun once. This, however, is not an exact number of days: one complete orbit takes 365.25 days. Instead of making days slightly longer to compensate for this (which would eventually cause them to no longer correspond to midnight and midday), the extra 0.25 days are saved up and after four years they make a whole day which is added on to the end of February (such years are ‘leap’ years). Centuries and millennia, a hundred and a thousand years respectively, are simply extensions of this concept.
On a technical note, the speed at which the Earth rotates and orbits varies slightly all the time due to gravity, tidal forces from the moon and the solar wind. These variations are tiny, but in the interests of accuracy the same rule is applied to them as leap years – every time the variation reaches a whole second, a ‘leap second’ is added on to the year (this happened 2008).
However, despite the radical increases in accuracy required today, all time standards used on Earth ultimately are defined by its rotation.
