What day is it?

VulcanSpirit
Richard & Alison Brunstrom
Sun 16 Sep 2012 17:42
In order to get to Tonga we have had to cross the Dateline - while at sea at 172.5 degrees West we went straight from Saturday to Monday, missing out Sunday entirely. As a result we went instantaneously from twelve hours behind the UK to twelve hours in front. Why?
Well, because the earth turns eastwards on its axis every twenty four hours it is necessary for the clock to be put backwards an hour every fifteen degrees of west longitude (because there are 360 degress of longitude, divided by 24 hours in the day) in order for the local time to be consistent with local daytime - and of course, forward an hour for every fifteen degrees of east longitude.
Zero longitude, which has to be somewhere but could in principle be anywhere, was set at Greenwich in London by interntional agreement (you can see the Prime Meridian set in the ground at the excellent Greenwich observatory) for two reasons: firstly Britain was the foremost world power at the time, and secondly because setting zero degrees in London meant that its opposite, the 180 degress meridian, cut largely through empty Pacific ocean and would not leave any large continents or countries inconveniently straddling both sides of the Dateline - for at 180 degrees it must perforce be one day on one side of the line, and another day entirely on the other side. 
The UK is on summer time at present, GMT/UT+1, while Niue was on UT -11 and Tonga (174W) is on UT +13 (the Dateline has been moved from its theoretical position of 180 E&W to 172.5W, to the east of Tonga, in order to put Tonga into the same day as its principal neighbour, New Zealand). But Tonga has decided to have the sun rise and set later than NZ (for the same reason that the UK has summertime), so that instead of its geographically 'correct' timezone of UT -12 it has moved 25 hours ahead to UT +13 thus placing it and us exactly twelve hours ahead of UK summertime. Clear?
Of course we haven't really 'lost' a whole day at all - we have been putting the clock back an hour every fifteen degrees since we left the UK, and will continue to do so as we go further west. We are now half a day ahead of the UK and will lose all of that in due course - but the sudden jump here, from one day straight to the day after tomorrow, can be a bit confusing at first!