A solar dial is a type of time switch used primarily for controlling lighting.
The benefit of a solar dial over a conventional 'on-off' time switch is the ability to 'track' the sunrise and sunset times for a particular latitude (which is specified when the unit is purchased). The solar dial 'adjusts' itself by a fractional amount each day, thereby ensuring that street lighting is switched on and off when required throughout the year. Many dials also have an additional 'part night' facility allowing for a switch-off in the middle of the night, and then back on in the morning if needed. This 'part night' option was widely adopted in the United Kingdom for street lighting in the 1970s and 1980s in order to conserve energy. Some solar dial switches have a clockwork or battery 'reserve' to maintain time accuracy in cases of power outage. If this is lacking, the switch would have to be reset every time the power fails, a labour-intensive task.