For centuries, the time of day was directly linked to the Sun's passage across the sky. This time standard is called "Local Solar Time" and is the time indicated on a sundial. The time such clocks would show would thus vary across the United Kingdom, as Noon is later in the west.
It is surprising the difference this makes. In total, the United Kingdom stretches 9.55 degrees in longitude from Lowestoft in the east to Mangor Beg in County Fermanagh, Northern Ireland, in the west. As 15 degrees is equivalent to 1 hour, this is a time difference of just over 38 minutes!
As the railways progressed across the UK, this difference became an embarrassment and so London or "Greenwich" time was applied across the whole of the UK. A further problem had become apparent as clocks became more accurate: due to the fact that, as the Earth's orbit is elliptical, the length of the day varies slightly. Thus 24 hours, as measured by clocks, was defined to be the average length of the day over one year. This time standard became known as Greenwich Mean Time (GMT).
The use of GMT has the consequence that, during the year, our clocks get in and out of step with the sun. The difference between GMT and the local solar time at Greenwich is called the "Equation of Time". The result is that the Sun is not always due south at noon – even in London – and the Sun can transit (cross the meridian) up to 16 minutes 33 seconds before noon as measured by a clock giving GMT and up to 14 minutes six seconds afterwards.
This means that sunrise and sunset are not usually symmetrically centred on midday and this does give a noticeable effect around Christmas time. Though the shortest day is on 21 December, the Winter Solstice, the earliest sunset is around 10 December and the latest sunrise does not occur until 2 January, so the mornings continue to get darker for a couple of weeks after 21 December whilst, by the beginning of January, the evenings are appreciably longer.
Greenwich Mean Time was formally replaced by Universal Time in 1928 but this was essentially the same as GMT until 1967. Then we changed the definition of the second...
Professor Ian Morison was speaking at Gresham College last monthReuse content