# Determining North-South Line Via Watch Method: Theory & Reason

I recently read that if you're in the northern hemisphere and have an analog watch, then you can point the hour hand at the sun and know that a south line lies between (bisection) the hour hand and the 12 o'clock position:

Apparently the trick works in the southern hemisphere as well and needs to be adjusted for day-light saving (image taken from Wikipedia):

I would like to know the reasoning behind this trick, and would like to know if the trick can be applied to tell the approximate time provided I only have a Brunton compass.

I made an illustration if it helps:

• You'll also have to correct for the Equation of Time and how far east/west you are of the defining meridian of the time zone, no?
– user21
Sep 3, 2015 at 14:46

When the Sun is on the meridian and is due South (local noon in Northern Hemisphere) your hour hand will point to (or close to 12) - assuming your local noon and civil noon are similar! (Daylight saving means 1pm fills this role).

The Sun crosses the sky in 24 hours, the hour hand travels round the watch in 12 hours. Hence the watch hour hand is twice as fast, so when the hour hand has reached 2pm, the Sun has only gone half as far (ie "to 1pm"), so if you point the hour hand towards the Sun in this case, the meridian is an hour further behind rather than 2 hours.

By the time we get to 4pm the Sun will have travelled "two watch hours" away from the meridian and so on.