Antique Pocket Sundial

[Hornbein]

A pocket sundial! from 1637. Such is not that unusual in the era before pocket watches. What is unusual is the map with labeled zones. What's that for? Correction for variations in the magnetic field? There's a built in compass so that's my guess. It shows they took this quite seriously. This must have cost a LOT of money.

 

[Ibix]

The map zones are the counties of England as they used to be (edit: and Wales, with the smaller ones in the south just treated as Glamorgan)(edit 2: actually that link shows modern regions of Wales - the counties in existence at the time of the sundial were these ones), labelled with their initial letters.