Sunday, 26 January 2014

Navigation and Time Zones

If you have ever flown across the pond to the USA then you’ll know that you end up with a long day – leave Heathrow at midday fly for seven hours and arrive around 2 pm local time with some five extra hours to go before bedtime. All because being further west on the globe New York is five hours behind UT (the correct terminology for GMT). It isn't too much of a problem, set your watch in the air above the UK, snooze for the flight,  stay up late when you land and soon your body catches on. So what happens to the clock change when you go more slowly, in our case at six or seven knots for the 2,100 nautical miles from Cape Verdes to Grenada? Having never sailed across the pond before I wasn't certain and nor was the skipper so it needed some investigation...

Time zones were agreed by a 1950’s convention to be centred on lines of longitude. Basic maths says divide 360 degrees (assuming the Earth is round not flat) by 24 hours and you get 15 degrees per hour. So starting at the Greenwich Meridian every 15 degrees you move West you need to subtract one hour to get the current time at that spot (hint the sun rises in the East and moves West). Therefore it is easy isn’t it to change the clocks? The aim is to keep midday on board ship approximately when the sun is at it's zenith (highest point in the sky). Well no the navigator still needs a bit more maths as the same convention states that though the time zone is centred on 0, 15, 30, 45 etc but it actually changes midway between these lines of longitude so the clock should be moved back by one hour at 7.5, 22.5, 37.5, 52.5 and 67.5 degrees West. Still with me?

Just to complicate things further nation states don’t have to abide by the convention, indeed some have never adopted it completely for example Delhi in India is five hours and thirty minutes ahead of GMT! Such is the case too with Spain or more precisely the Canaries. The islands are located either side of 15 degrees west so firmly in UT-1 however mainland Spain uses (rightly or wrongly as covers more than one time zone) a single time zone UT+1. All this means that if the Canaries were in the “correct“ time zone for their latitude at best there would be two hours difference and in the summer with Spanish daylight saving* the difference would be three hours. For convenience of business and government, the Canaries are actually always on UT making the difference only one hour in winter and two in the summer. This in turn means for us that during the 850 nautical mile trip south to the Cape Verdes (in time zone UT-1) Temptress needs at some point to lose an hour despite remaining in the same longitude range! We’ll probably do to that the ships clock as we leave Las Palmas harbour, again for convenience, much like crossing the English  Channel to France except the voyage is much further.

As for Grenada, the eastern Caribbean is in zone UT-4 meaning we need to lose another three hours as Temptress crosses the Atlantic from the Cape Verdes. If we did this in one go at the start of the trip our poor bodies would complain at the delay to meal times plus the dark mornings and light nights initially would not be conducive to sleep adjustment. Therefore the sensible approach is to change the clocks each time we sailed across one of the time zone boundaries ie latitudes (22deg 30W, 37deg 30W and 52deg 30W ) but with the slight adjustment of delaying the change to coincide with the next main meal of the day which is when the watches will also move (another subject entirely but mainly to avoid someone getting the same dead of night hours every night for the whole trip). We plan to eat lunch or dinner (depending on your origin) sometime in the early afternoon whilst it is still light enough to see what is being cooked/eaten/washed up so unlike the end of British Summer Time which happens around in the wee small hours giving you an extra hour in bed we will be extending our lunch break every few days!

Haring through some rough but inaccurate maths based on distance and speed also gets me as Temptress' erstwhile navigator to the approximation that the ships clocks will need changing every 4 to 5 days. However this estimation doesn’t take into account the wind speed or the course we actually manage to sail so the longitude change is what we'll be monitoring rather than distance covered or time at sea.


For more on time zones and navigation with a neat diagram showing how Time Zones work refer to Tom Cunliffe’s Celestial Navigation book.

And if you ever need to know what time it is somewhere else in the world then try the useful timeanddate.com


* Daylight saving is only useful in the higher latitudes where there is a marked difference in the length of day light hours between winter and summer – why is a topic for another day!

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