What day is it?
VulcanSpirit
Richard & Alison Brunstrom
Sun 16 Sep 2012 17:42
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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!
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