Hortsmann solar calendar time switch manufactured for latitude 56 degrees: an electro-mechanical time switch self-adjusting to sunrise and sunset times (GMT). There is an advance and retard mechanism which allows the switch to operate at preset times before or after sunset or sunrise.
A device like this was used in nearly every lamp column in Edinburgh up until 1975 when they began to be replaced by photoelectric cells: they were still being used in 1995 in a small number of older columns. They were generally regarded as having a high degree of accuracy but there were cost and practical implications of their continuing use: (1) every switch needed to be reset following any power cuts of significant duration, (2) watchmakers were permanently employed to maintain and repair the mechanism. These mechanical time switches have now been largely superseded by electronic switches. They are sought after quite avidly by some collectors.