London dial

London dials were originally engraved by scientific instrument makers.

A gnomon or style is set to point at the celestial north pole, the shadow of the Sun is thrown onto the dial plate and will appear at the same position each day of the year, and this position can be calculated using trigonometry, or drawn using geometric construction.

[a] A London dials show hour lines, some show half-hour markers, the quarter hours and some divisions representing half-quarters (7 and a half minutes).

The equation of time is shown as a three rings, showing the words WATCH SLOWER, WATCH FASTER, the months and dates and the minutes of time of variance.

[2] A horizontal dial, takes the equiangular hour lines of an equatorial dial and projects them onto a plane oblique to the style, the markup simulates this transformation.