Longitude

Dava Sobel, in her recent best selling book Longitude, highlighted the inter relationships between some of these elements. Particularily, longitude, time and the sun.

Although something of an oversimplification, it is a truism that if you know any two of the components of this equation, you can work out the third. Add in latitude and the date and you can do it exactly - and that is what the heliochronometer does. It is, in effect, making allowances for the vagaries of the earth’s movement throughout the year and for the regular effect that these have on the apparent position of the sun.

The potential to extract exact time from a really accurate sundial has, of course, been known for years and many, better quality, dials are accompanied by a Table which allows one to make the necessary corrections for each day of the year. In doing this automatically, the heliochronometer is reaping the rewards of the evolution of sundials over the last three thousand five hundred years.

See also:

• A history of the heliochronometer
• The equation of time
• Full installation instructions for the Gunning Heliochronometer

See the full diagram of the earth's movement throughout the year