Article of the month February

Clock with equation-of-time dial

A pole style sundial on a smooth wall has straight hour lines.
In former days, local apparent time was the usual time system.
After the introduction of time zones, a longitude adjustment was sometimes applied to the hour line pattern in order to be able to read civil time.
To arrive at clock time, the equation-of-time had to be taken into account.
To this end, a table or graph was often placed near the sundial.

But the photograph above shows a clock where the reverse was done.
The watch face reads civil time, and a separate dial shows equation-of-time.
Together with the value for longitude adjustment, this allows one to calculate local apparent time.