TONIGHT, probably before we go to bed, we will advance our clocks and watches by an hour, so that one o'clock will suddenly be two. Both times, however, are in a sense fictitious, adopted by convention for convenience: the true time, will be something else again, depending on where any individual may he.
One of the first ways by which man learned to estimate time was by observing the passage of the sun across the sky. Time assessed in this way at a particular spot is called Local Apparent Time, or LAT.
Now one might presume that the length of a day measured in this way - or indeed, in any other - is precisely 24 hours, but this is not so exactly. If you measure the days accurately by the sun, they turn out to differ slightly in length, being at some times of the year a little longer, and at other times a little shorter than the precise 24 hours; you will find that in November the sun reaches the noon position LAT about 16 minutes earlier than it would if the days were all of equal length, while around this time of year, it is nearly 15 minutes late.
These variations arise for a number of astronomical reasons, one of which is that the earth moves faster in its orbit at certain times of the year, when it is closer to the sun, than it does at others. To avoid the practical inconvenience of a clock which varies with the seasons, we pretend that there is a fictitious sun in the sky which revolves around us at a conveniently constant rate - a rate equal to the average speed at which the real sun appears to move in its orbit. Time measured by this average - or mean - sun is called mean solar time, and mean solar time at any particular spot is called Local Mean Time (LMT).
In olden days, each town kept its own local time, as near to Local Mean Time as they could manage, and indeed with both travel and communication conducted at a more leisurely pace than they are, now, the inconvenience was minimal. During the last century, however, the development of the railways, inter alia, made it necessary to introduce some order into these affairs, and the Statutes (Definition of Time) Act passed by the Westminster Parliament in 1880 decreed that Greenwich Mean Time should be the official time for Britain. It also gave us on this side of the Irish Sea a measure of independence by putting Ireland in a separate time zone: Dublin Mean Time, which applied throughout the island until 1916, was 25 minutes behind GMT.