If you are in the military, it would be Zulu time.
Yes, I should have been more careful - indeed as I typed the sentence the niggle knomes at the back of my brain went “isn’t Portugal in the same zone as us?” and I ignored them. :smack:
To throw another good book into the suggestions, Derek Howse’s Greenwich Time and the Longitude (1980; Philip Wilson, 1997) is getting rather old, but is rightly kept in print by the National Maritime Museum at Greenwich. Not as detailed as Galison or Blaise in their particular areas, but he tells the overall story well.
[QUOTE=bump]
[ul]
[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.[/ul][/li][/QUOTE]
Having read both Longitude and Time Lord, I think you’re a bit off in the summary.
First, the solution of the longitude problem was in the 18th century, during George III’s reign. There were two competing approaches to determining longitude - by astronomical observations, and by chronometers. The Royal Astronomers of the day were actually quite hostile to the chronometer approach, influenced in part by ideas of class and classics. Gentlemen did astronomy; rude mechanics made watches. The fellow who successfully built the chronometers that solved the problem (name escapes me at the moment) had to fight hard for his approach, over the opposition of a series of Royal Astronomers. He built them using his own money, competing for a monetary prize, and by the account in Longitude, was never properly compensated for his work.
Second, the solution of the longitude problem and the choice of a universal prime meridian did not happen at the same time. The choice of Greenwich occurred a century later.
Unlike latitude, which has easily determined natural reference points (the equator and the poles), longitude is purely artificial and can be calculated from any arbitrarily chosen prime meridian. Once there was a way to determine longitude, each of the major countries started using their own prime meridian to determine longitude. The French calculated longitude based on the Paris prime meridian, for example. So each nation’s navigational charts were different, making it necessary to do conversions when trying to communicate one’s position at sea to a ship from another nation.
The solution was the adoption of the Greenwich longitude as the prime meridian, which occurred at the Washington Conference in the 1880’s. Britain and the U.S. formed a block in favour of Greenwich as the prime meridian, and were successful in having that adopted by international convention, over France’s objections.
tomndebb already mentioned it some, but in the GMT system, each minute of the day had exactly 60 seconds. It was syncronized with the (variable) rotation of the Earth by making some seconds longer than others.
Since the definition of the “second” was pegged to the cesium atom many years ago, the second is precisely defined. Variance in the Earth’s rotation is accounted for by adding “leap seconds” occasionally, so that sometimes the last minute of the year will have 61 seconds.
GMT and UTC can vary from each other by as much as a second.
I’m having trouble picturing this. Why should an elliptical orbit affect the length of a day? It would affect the rate of the earth in its orbit, not the rate of rotation, no?
While rotating once on its axis, the earth also moves 1/365th (roughly 
) around the sun. So it takes that much more than one rotation for the sun to appear in the same position once more. With an elliptical orbit, some points in the orbit require less than 1/365th extra rotation, and others require more.
That would be John Harrison. The whole story is available at this link.
The Earth’s orbit is elliptical — but the more important effect is: the Earth’s angular velocity around the Sun is not constant. When the Earth is closer to the Sun, it moves a bit faster; when it’s further away, it moves a bit slower.
Since the Earth’s rate of rotation is constant (or near enough, for this consideration), that means the Sun “speeds up” and “slows down” during the course of a year, as measured by a clock on the ground keeping regular time. Over the long term, the average length of a day is indeed 24 hours — but the actual length of any particular day will be different from this in general.
A friend of mine, who knows more about the French than I do and speaks the lingo, told me that the French refer to ‘Le Havre’ time rather than ‘Greenwich Mean Time’. They are referring to the same meridian and it means the same thing, but for reasons (I assume) of national pride they use the French place name rather than the English one. There is no observatory or physical mark of the meridian at Le Havre.
However, I tried Googling on ‘Le Havre time’ and didn’t come up with much to substantiate this suggestion. Maybe my friend is either wrong or out of date.
I never heard about this “Le Havre time” in my whole life. Your friend must be very out of date.
It produces one of the most unequal Googlefights I’ve ever seen
I worked for four years in France, with close ties to astronomers at the Paris Observatory in Meudon, and never once heard of “Le Havre time” as an alternate to GMT (and trust me, they would have used it jokingly if there were any precedent, in the cause of franco-britannic rivalry), so I have to concur with clairobscur that your friend is misinformed.
I can, however, think of a possible source of the confusion. Le Havre was the main French port for transatlantic steamships, like Southampton was for the UK and New York for the US. So, there might have been a tradition of referring to “Le Havre time” (as opposed to “New York time”) while on board ship, so everyone knew what time to show up for cocktails with the captain.
That’s an excellent suggestion for what this means - it seems perfectly logical to imagine one of those lines of clocks, labelled “New York” - “Southampton” - “Le Havre” - “Cairo” - …"
I would very much recommend visiting the Royal Observatory if you are ever in London, BTW.
A great explanation about the Equation of Time and Analemmas is here.
We had a similar discussion on this in a previous thread.
Another tentative explanation for ianzin’s friend’s claim. The key meeting that founded the present international arrangements was the International Meridian Conference, held in Washington in 1884. Needless to say, the French abstained in the vote on the resolution that Greenwich be used as the Prime Meridian and then ignored the fact that the Conference had voted in favour of this. Defiantly - and by then there was a degree of symbolism in legislating it, since this was already the de facto arrangement in the country - they then passed a law in 1891 enforcing Paris Mean Time throughout France.
Evidently this proved unworkable. In 1896 a bill was proposed to switch the country to GMT. In a fairly transparent bit of Gallic national facesaving, this was amended to define the new standard meridian to correspond to Paris Mean Time minus 9 minutes 21 seconds. The whole process got hung up, but a version of the bill was finally passed in 1911 and this became the legal time system in France until they switched to UTC in 1978.
Of course, PMT - 9’ 21" was equivalent to GMT for most practical purposes. I doubt anyone remotely knowledgeable was ever taken in and I can see someone dubbing this “Le Havre time” as a joke.
As Chronos already mentioned, the axial tilt also figures into it.
Trivia: the Greenwich observatory is no longer in Greenwich. The Old Royal Observatory is there, but it as a museum.
History from Wikipedia
1948 Astronomer Royal moves to Herstmonceux.
1957 Royal Observatory completes its move to Herstmonceux, becoming the Royal Greenwich Observatory (RGO). The Greenwich site becomes the Old Royal Observatory.
1990 RGO moves to Cambridge.
1998 RGO closes. Greenwich site becomes the Royal Observatory, Greenwich, and is part of the National Maritime Museum.
Brian
The light dawns. Thanks, **Yeticus **. GorillaMan too.