09/09/2026
I recently came across this interesting sundial with two faces, one for each half of the year, but even after adjusting for the fact we are still on BST, it didn't quite tell the right time........
So I looked into it, and for next time you are out exploring, here is how you read a sundial. In brief, the needle (or gnomon) must point true North, & the sundial should be set up for the longitude of your location. If 12 o'clock is in the middle of the dial, it hasn't been set up & you will need to make a longitude adjustment: add 4 minutes to the time indicated by the sundial for each degree you are west of Greenwich. Exeter for example is 3.5 degrees west so we add 4x3.5 or 14 minutes. Almost a quarter of an hour is quite a lot, isn't it!
A final corrective step is now needed because a sundial reads Local Apparent Time. We Google the "equation of time graph" to see how many further minutes need to be added or subtracted for our particular time of year. This is necessary because the earth's annual path around the sun is not a perfect circle but an ellipse, & because the earth is tilted on its axis (which is what gives us the seasons). At the beginning of November for example, this second step would result in a further 17 minute correction.
If you've followed all that, you will now get sundial readings which make sense.
