Why is everywhere apaert from England judged on the time in England plus/ minus a few hours? Shouldn’t those places have their own mean time? Foe Example shouldn’t Syndney have Sydney mean time?
someone please reply
Mark Question
England does use GMT. There’d be little point in having more than one mean time as that would defeat the purpose of a mean time.
The purpose is to prevent confusion. If you say GMT -7, everyone knows what to measure it against. If you say Sydney Mean Time -7, everyone would have to recalculate the time in that system. The location of GMT was arbitrary (it could have been any point in the world), but in order for everyone to know the situations, one location had to be chosen.
Greenwhich 1 day is a lie.
In the period when the idea that different places had different sun times became important, the premier astronomical site in the world was the Royal Observatory at Greenwich. The line running from North to South Pole through that establishment was deemed the Prime Meridian in consequence, since the majority of chronometers would be set based on Greenwich time (not Greenwich Mean Time – they’d be British ships setting out to sea with chronometers set to the Royal Observatory’s time, important in determining longitude. (Rumor has it there’s an excellent book explaining this.)
When the world, less a few places that set their own exceptional time standards, was divided into 24 time zones, Greenwich Mean Time was used as the foundation from which the others were described. Since there needs to be a zero point, it had historical precedence for being that. But it would be equally accurate to consider Auckland Standard Time, 12 hours off from Greenwich, as a basis, or whatever other zone you chose. It’s just convenient to figure based on plus or minus what was historically the standard from which to figure, and no one has advanced a good reason to change it.
Technically, one can define a mean time for any location on the planet. Furthermore, this is what people once did do. Each town based their clocks on the apparent local noon. Since the moment of noon at any location is something that varies continuously round the planet through the course of the day, this meant that these local times were different for different longitudes.
The practical problem arises when you want to compare these different definitions during the hurly-burly of daily life, as began to happen in the 19th century with the advent of railways and telegraphs. For instance, while it then became possible to travel between them in less than a day or send a message between them almost instantly, London and Plymouth have local mean times that differ by about 16 minutes. What time should apply on the timestamp of a telegram between them? Or on the timetable used by a passenger traveling from one to the other?
Initially there was much confusion. Different railway companies used different rules. Some towns insisted on keeping their public clocks to local time, others switched to London time. (And even then there could be ambiguities - there’s about 23 seconds difference in local time between St. Paul’s Cathedral in the City and that at Greenwich.) The end result, however, was that the whole of the UK and Ireland came into line with GMT. That way, everybody at least knew what anybody else in the country meant when they mentioned a specific time.
To cut a long story short, the international system became a generalisation of this.
There’s a nice pair of old photos reproduced in Einstein’s Clocks, Poincare’s Maps (Sceptre, 2003, p223-4) by Peter Galison of a public clock tower in Geneva. The first shows it c.1880 when there had to be three dials: one showing Paris Mean Time, another Geneva local time and the third Bern local time. There’s now only one dial because everybody on continental Europe is now in the same time zone. It’s all much simpler that way.
As to the “mean” part: Astronomical noon is defined as the time (in any particular location) when the Sun is at its highest point in the sky. But due to the eccentricity of the Earth’s orbit and the tilt of its axis, it’s not exactly 24 hours from one astronomical noon to the next: Sometimes it’s a little bit longer, and sometimes it’s a little bit shorter. So if you want to divide a day up into 24 chunks which are always exactly the same length, then you have to define noon as being the mean (or average) time at which the Sun is at its highest. So 12:00 GMT on one day is always exactly 24 hours before 12:00 GMT on the next, and if you’re in Greenwich, the Sun sometimes is at its highest a little before 12:00 GMT, and sometimes a little after, but it averages out to 12:00 GMT exactly.
Let’s clear up a few things.
A) “Mean time” means that the time is averaged out over the year. Because the Earth’s orbit around the Sun is an ellipse, the days are slightly less than 24 hours part of the the year, and slightly more for the other half.
B) The UK does not use GMT, because the UK has Summer Time, just like the US. British Time is the same as GMT in the winter, but an hour ahead of GMT during the summer.
C) Technically, GMT is actually obsolete. On an atomic clock, GMT has never had any leap seconds. It was replaced by UTC, which does have leap seconds.
Dang. I was going to mention Einstein’s Clocks, Poincaré’s Maps.
So let me throw in a weird little obscure book, Time Lord : Sir Sandford Fleming and the Creation of Standard Time [BARGAIN PRICE], by Clark Blaise.
Bargain price? Good stuff.
Anyway, Galison’s book covers the ground from a more European perspective, with the emphasis on the conferences there and on the battle between France and Britain as to who would get the zeroth longitude, while Blaise’s book goes more into the North American need for a standard time and the Washington international conference that ensued.
Allow me to second the recommendation of Time Lord. An iteresting man in interesting times.
This article is misleading. The meridian line was just used as a pretext for a major feast (which was a success), and in particular a giant picnic, from the north to the south of France.
I didn’t hear once the issue of the choice of the meridian line being mentionned at this time.
As John W. Kennedy pointed out earlier, GMT is obsolete. Anyone knowledgeable enough to be reading the SDMB should use the term Coordinated Universal Time, or UTC. And it’s really different - not just a name change.
Here’s the condensed reason as to why GMT is the reference:
[ul]
[li] Without accurate timekeeping, it was almost impossible for navigators to determine a ship’s longitude accurately.[/li][li]The British, having the biggest maritime presence in the 19th century, put a lot of money into developing said chronometers.[/li][li]The Royal Astronomer and his observatory at Greenwich was charged with the chronometer projects and the determination of longitude.[/li][li]Since the Observatory at Greenwich was where the master chronometer was set, it became both the Prime Meridian and the reference time when the longitude problem was solved.[/li][/ul]
That’s how I remember it from teh Royal Observatory at Greenwich tour and as reinforced by the previous posts.
The excellent book is Longitude I believe. However you may also choose to read Conrad’s The Secret Agent wherin the Royal Observatory becomes the subject of a suicide bomb attack.
[nitpick to an otherwise excellent post]
Geneva, Paris, and Bern are indeed all in the same time zone now, but that’s certainly not true of “everybody on continental Europe”. Portugal is at GMT+0, countries from Spain to Central Europe are at GMT+1, countries on a meridian from Finland to Greece are at GMT+2, and the European part of Russia includes parts of three other time zones (GMT+3 to GMT+5). See here.
[The GMT-relative times listed above are winter only, and don’t account for daylight saving time.]
[/nitpick]
(I realize that you undoubtedly already know this, bonzer, and it was probably just a slip of the keyboard, but others may take your “continental Europe=1 time zone” implication at face value, and this is GQ.)
Yes, there are significant differences between the two, but in normal conversation with normal people (meaning, those who don’t read the Dope) you will have to develop a tolerance for hearing “Greenwhich Mean Time”, which, for most peoples’ purposes, is close enough. So it’s quite fair (in my view) to mention that many people hold the terms to be synonymous, even if technically they aren’t.
Why doesn’t somebody explain what exactly the differences are between GMT and UTC?
Because that would be telling.
At its most primitive (and, therefore, not wholly accurate), explanation, UTC allows for leap seconds based on a Cesium clock while GMT is based on celestial observation.