Finding North without a Compass (3/5) – With a regular watch

With a regular watch (graduated from 1 to 12)

Orient the watch so that the small hand is pointing in the direction of the Sun, while keeping the watch flat. To improve the quality of the reading, you can plant a stick in the ground and orient the watch so that the shadow of the stick merges with the small needle.

Locate the angle between the small hand and noon on the dial. Divide this angle in two (for mathematicians: draw the bisector of the angle between noon and the small needle) and draw the line to the opposite side of the dial: this is North.

Be careful: if you are in the southern hemisphere, it is the opposite. The bisector indicates North, and the line to the opposite side of the dial gives South.

How does it work?

By definition, at noon, the Sun is in the South. So when the hour hand indicates noon, if you point it towards the Sun, the opposite direction on the dial is North. What happens an hour later? The hand has moved one-twelfth of a turn (there are 12 digits on the watch screen), while the Sun has moved a 24th of a turn (it makes one turn in 24 hours). This is why the angle between the small hand and the midi on the watch screen must be halved.

In practice, the main difficulty of this method is to set your watch to solar time, even though it indicates legal time. To do this, in France, you have to set your watch back two hours in daylight saving time and one hour in winter.

NB: there is another discrepancy between legal time and solar time: the equation of time. But we can disregard this for this exercise, because the difference amounts to a maximum of 20 minutes, which corresponds to a difference of a third of the angle between two hour graduations on the watch face, i.e. 30°, which are reduced to 15° when the bisector is drawn (360° divided by 12, divided by 2).