Hi, this has come up recently at my work as this means nothing in French in geographical terms, as there is only the Northern and Southern Hemisphere for them. And yet, while agreeing at first, I did a random search and it is used quite frequently to refer to North and South America as well as the Caribbean nations.
Does the term instantly encourage derision as the French think?
In English it is used officially for offices and designates what I just said.

Just give me your impressions, and thanks in advance!

Good question, Bruyere. We in the Western Hemisphere don't think about it much But, yes, the hemispheres are most conveniently divided at the equator rather than at 180 degrees west longitude.
Don't know where the "daffynition" came from originally but probably was coined just to irritate the French.

"Eastern Hemisphere" and "Western Hemisphere" are certainly well established terms in English, though a quick check in my English-French and English-German dictionaries suggests that other language communities probably only recognize Northern and Southern.

When we talk about the Northern and Southern Hemispheres, we can define them exactly by reference to the Equator. Also, there are easily observable basic differences between North and South. It's summer at present in Australia but it's winter here in Europe. I've also been told that bath-water going down the plug-hole will swirl clockwise in one hemisphere and counter-clockwise in the other, but I can't remember which is which.

But Eastern and Western Hemispheres are much vaguer terms. The OED and Chambers agree that the Eastern Hemisphere is the "half" that contains Europe, Africa and Asia and the Western is the "half" that contains the Americas.

These definitions, and the quotations in the OED, suggest that the terms came into use in the wake of the rediscovery of America by Europeans, and that they mean little more than "the East" and "the West", with the dividing line being very vague. The Americas were, as far as people knew, "half a world away" from Europe, and it was natural for Europeans to think of the world as being divided longitudinally somewhere off the Atlantic coast of Europe and Africa (the division obviously doesn't coincide with the meridian of Greenwich, otherwise I'd be moving between hemispheres when I travelled three stops westwards from home on the London Underground). Is Australia in the Eastern or Western hemisphere? I don't know, probably because Australia was unknown to the persons who first used the terms.

In a sermon in 1624, John Donne spoke of "The Western Hemisphere the land of Gold and Treasure; The Eastern Hemisphere the Land of Spices and Perfumes".

The differences between the "Old World" and "New World" were probably big enough to justify thinking of them as separate hemispheres when the Americas were rediscovered, and we may still think that there are considerable cultural differences.

But the differences between the vaguely defined Eastern and Western Hemispheres are not of the same order as those between the Northern and Southern, and so the French may be justified in refusing to believe in "Eastern" and "Western" Hemispheres. Indeed, if they can read the magnificent seventeenth-century English of Sir Thomas Browne, they can find the arguments against such an idea in Chapter 7 of Book 6 of Browne's "Pseudodoxia Epidemica". I'm eternally indebted to Bruyere for raising this topic, because it's led me to find a complete text of Browne's work on the web at http://penelope.uchicago.edu/

Thanks so much for your answer, I hadn't seen it!
You see, one of the main reasons I decided to write a book about French and "Anglo-Saxon" relations (don't worry, it's not nearly as heady as it might seem) is that whenever there is a lack of comprehension like that, there is always a reason!
It's normally not just the French being stubborn as usual.
And this was someone who spoke English very well and had studied in the States, so when she was making fun of the person who'd written it, I asked her why, feeling a bit sheepish! I told her that it didn't sound out of place, especially since it was referring to the entire continent and the Caribbean islands in this context.
And she still found it ridiculous.
The French take their geography very seriously, probably as much as the Germans. How many times did I play Trivial Pursuit and found myself surprised at their knowledge of American geography!
They probably know more State capitals than many Americans do!

What I normally do on a term or something is do an internet search and see how many hits I get with the one term, and take the happy medium.
So this is why when I saw that I'd been right this time, I wanted to ask for a consensus of some sort.

If one were to interpret 'Eastern' and 'Western' Hemispheres with absolute pedantry then the result for both England and France would be interesting, to say the least.

Well I knew I'd hit pay dirt when the lady looked at me like I was an idiot...and then the English guy heard it and kind of laughed but I could see he didn't see much wrong with it.
Then when I saw respectible offices called that I knew it was one of those French things!