Torquetum-type sundial, English, late 17th century.
© Science Museum / Science & Society Picture Library
This sundial was constructed to make celestial measurements according to a number of different systems. The lower dial, set in the equatorial plane, can be used as an equatorial sundial while the upper dial tells the time according to the stellar coordinates. The sundial is constructed so that it can show both solar and sidereal time. The levelling screws, compas and plumb bob on the base are made to ensure that the instrument can be correctly aligned before any measurements are made.