This clock was designed by John Flamsteed, first Astronomer Royal, to show the time in degrees, minutes and seconds of arc. In astronomy it is often more convenient to express time in this way. The earth turns through 360°on its axis every 24 sidereal hours.

24 hours = 360°
1 hour = 15°
4 minutes of time = 1°
4 seconds of time = 1' of arc
1 second of time = 15" of arc

The clock reads as follows: small window right of centre – tens of degrees; 'minute' hand – degrees, with 10-minute of arc subdivisions; 'second' hand – minutes of arc with 10-second of arc subdivisions.

The pendulum is 17.3 inches long instead of the usual 39.1 inches and so beats 2/3 second of time (or 10 seconds of arc) instead of 1 second of time. To convert the clock to read normal hours a pendulum of 39.1 inches can be fitted and the small dial behind the aperture reversed to show I to XII hours. The clock then shows hours and decimal parts of an hour.

In practice this system was not very useful for astronomers and it is believed that only two 'angle clocks' were ever made: this one, and one by John Arnold, currently on display in the Meridian gallery at the Royal Observatory, Greenwich.