THE EQUATION OF TIME

IF, during it's annual orbit of the sun, the earth did it's daily spin around an axis which was at right angles to the plane of that orbit, - and - IF the orbit was circular, - then every day would be the same as the last. There would be no seasons. The inhabitable portion of the earth would be much reduced and Life would be very boring. But the task of the Sundial would be easy. There would be no Equation of Time.

HOWEVER, it is not like that! The earth's orbit around the sun is elliptical, not circular - and the axis of it's daily orbit is inclined at 23.45 degrees to the plane of that orbit. These factors combine, on the one hand to give us the seasons, whilst on the other, they present the sundial with the problem of an irregularity in the apparent movement of the sun. Unless corrected, only on four days of the year will sundial time coincide exactly with the more regular 'clock' time.

The formula for this correction is the Equation of Time and, uniquely, the Heliochronometer applies it automatically. Also automatically, it caters for the time variations due to longitude and national daylight saving measures.

See also:

• A history of the heliochronometer
• The importance of Longitude
• Full installation instructions for the Gunning Heliochronometer