Back in the day, practical things used to be taught. One of the things I learnt was how to navigate using my wristwatch. The first time I piloted a single engine aircraft across the Atlantic even though we had accurate GPS navigation on board, I still used it as a gross error check. So what use is it when out in the caravan, motorhome or even tenting?
Well, we always try to have the sun on the awning side of the caravan, so choosing a pitch with the right orientation is a ‘must’
So how do we do that with a wristwatch?
Well, the earth takes 24 hours to rotate, and the day is divided up into even amounts – hours. So at any given time of the day we can work out where the cardinal points are on a compass from the time and the position of the sun – easy!
OK everyone knows from their school days that at 12 noon the sun is due south if you are in the northern hemisphere, north of the equator. So if we take that as a starting point we can start to understand how to use your watch as a compass. In the drawing below, at 12 noon we point the hour hand directly towards the sun and the 12 o’clock position is due south.
Right we hang about on the same spot for two hours until 2 o’clock and the sun has moved across the sky. So now we need to adjust for the sun’s movement. We look at our watch and this time we need to split the angle between 12 and the hour hand which is pointing at 2.
Huh.. I don’t get that?
Remember it takes the earth 24 hours to rotate and we are using a 12 hour indicating watch, not a 24 hour indicating watch. So we have to divide the hours making each “watch hour division” represent 2 hours of earths rotation. The sun passes across the sky at 15 degrees per hour (360 / 24 = 15) the angle between each hour division on a watch face is 30 degrees, so we have to divide this in half so it represents 24 hours on your watch face. Or to put it another way:- your hour hand is moving twice as fast as the sun, as it has to go round your watch face twice in a day while the sun only ‘goes round once’ in a day.
Lining up the dividing line between 12 and the hour hand towards the sun means that the 12 o’clock position is now pointing due south.
Real world example.
It’s just gone 3 o’clock. So I divide the angle between 12 and the hour hand (in this case it’s between 7 and 8 minutes past) and point the imaginary (red) line towards the sun. The 12 o’clock position now points to the southern cardinal point. With a couple of goes, you can do it quite easily.
How accurate is it?
Well the sun passes across the sky at a rate of 15 degrees per hour (360 / 24 = 15) so even with daylight saving you are only going to be about 15 degrees out which is the angle between 0 and the 2.5 minutes mark on your watch face.
With a little skill, you can work out where the sun will be. Looking at the drawing above, I can tell that the sun will be in the 3 o’clock position when it’s 6 pm…. so we can get the caravan just in the right position to sit in the evening sun with our G & T’s.
Hope this was a bit of useful fun.