Wednesday, 29 February 2012

A Brief History of the Measurement of Time

It’s Leap Day, and if you’re not busy proposing to your fella or counting the number of bonus attoseconds, what better way to spend it than considering where the day came from, and how indeed we have come to measure the unmeasurable concept of time?
'Time stands still for no man'

You may already know that the leap day occurs in the Gregorian calendar because the Earth takes 365 days and six hours to make a complete revolution around the sun, and that the accumulated six hours make up that extra day every four years. But did you know that years that are evenly divisible by 100 do not contain a leap day, unless they are also divisible by 400? This means that there was no Leap Day in 1900, but there was in 2000.

The Gregorian calendar, a solar calendar, was introduced in 1582 by Pope Gregory XIII in an effort to correct the inaccuracies in the Julian calendar that assumed that the time between spring equinoxes was 365.25 days, when in fact it is roughly 11 minutes shorter. The spring equinox, which determined the timing of Easter, thus kept creeping forward, and so the Roman Catholic Church sought a way to stabilise time.

But, of course, although an internationally-recognised system, the Gregorian calendar is not the only one concocted in history still in use today. Consider the Islamic calendar, which is lunar and based on 12 months in a year of 354 or 355 days and therefore has a discrepancy of 10 or 11 days compared to a solar calendar such as the Gregorian calendar. This means that the Islamic calendar does not synchronise with the seasons. One of the main challenges of this for practising Muslims here in the UK can be the moveable nature of the month of Ramadan. Fasting during daylight hours in mid-summer is testing, to say the least. And it’s hard to plan ahead with the Islamic calendar as one month may only begin when the crescent moon has been sighted for the first time shortly after sunset by one or two trustworthy menfolk testifying before a committee. The combination of further factors, such as the moon setting progressively later than the sun the further west you travel, means that the beginning of each month differs from one Muslim country to the next.

Once you’ve decided which calendar to follow, you need to select a method for measuring the days therein. Where did that all begin? It’s those canny Egyptians we can thank for being the first to organise the day. They arranged a 24-hour day which divided the night into 12 hours, tracked by the position of the stars in the sky. Day was divided into 10 hours, with the extra two hours accounted for by the hours before sunrise and after sunset. They used shadow clocks or sundials to tell the time during the day. It was their obvious design fault of not working at night that gave rise to the invention of the clepsydra or water clock where time was measured by the regulated flow of liquid. Again, the design was not perfect as the rate of flow varied according to the temperature and so sand clocks or hourglasses came into existence.
Egyptian water clock

Time measurement moved on in the eleventh century when a Chinese inventor came up with a mechanical clock, the technical challenge being how to create a wheel of a manageable size that would turn continuously at the same speed as the Earth. While the first model of a mechanical clock was created in 725 AD, it was Su Sung who in 1092 unveiled his “cosmic engine” to the world—a 35-feet-high monster incorporating a sphere for observing the stars and a chain drive driven by dripping water.

And it’s Galileo Galilei we have to thank for the pendulum clock. Standing in Pisa Cathedral one day in a reverie, he noticed that no matter how long or short its arc, the chandelier there took exactly the same length of time to complete a swing. And so his observation inspired the invention of the pendulum in the late sixteenth century, an amazing achievement when you consider that they are still widely-used today.

Quartz clocks, where quartz crystal regulates an electronic oscillator to keep time, came into use in the 1920s. Atomic clocks, using the spin property of atoms as their mechanism, emerged in the 1950s—and these are accurate to seconds in millions of years.

And, of course, we must also pay homage to the Global Positioning System (GPS), one of several internationally-recognised systems to keep tabs on time. The space-based satellite navigation system was developed by the US in 1973 to provide location and time information and is a vital component of many a Smart phone today. (We have President Ronald Regan for gifting GPS to the world in the 1980s. He decreed that it should be available for all after the shooting-down of Korean Air Lines Flight 007 after in strayed into the USSR’s prohibited airspace for lack of decent navigation.)

'The Persistence of Memory' by Salvador Dali

Greenwich Mean Time (GMT) is another household name worth exploring, most especially as it brings us neatly back to the Stephensons (see last week’s blog post). The system refers to mean solar time at The Royal Observatory in Greenwich, London, and is the official time during winter months in the UK. GMT had traditionally been used by British mariners to calculate their longitude from the Greenwich meridian, considered to be longitude zero degrees. But it was the expansion of the railways in the mid-eighteenth century that really put Greenwich on the map. Before “railway time”, based on GMT, was introduced in 1840, time was set by referring to the position of the sun from town to town. This was not a problem pre-railways as journeys took longer and the traveller could adjust his or her timepiece every now and again without too much bother. With the much faster journeys afforded by trains, a standard time was needed to bring schedules in line. The Great Western Railway Company was the first to use GMT as its benchmark in 1840, and by 1848 the other companies had followed suit. GMT caught on in many other areas of life, and by 1855 it was reported that 95 per cent of towns and cities had adopted it.

I think I can unashamedly say that I have now come full circle on this very brief history of time. I shall now undo all the above talk of how to measure time with a quote from Benjamin Disraeli:

‘But what minutes! Count them by sensation, and not by calendars, and each moment is a day.’


  1. Replies
    1. Time flies when you're having fun, as they say, David!

  2. A lovely informative post. We used to have clocks taken apart 'to find the tick' by our young children... these days I marvel at being ten minutes ahead by the kitchen timepiece, yet ten minutes late by the time I reach the hall :-)