Longitude Correction
To make a dial that correctly tells standard time you will need to know your latitude for dial construction and longitude to determine the correction (time zone offset) between your dial and the time zone meridian (usually set every 15 degrees of longitude).
IMPORTANT Some maps will indicate longitudes in North American as negative, indicating that they are west of Greenwich. Other maps will indicate longitudes in North American as postive, declaring them "west longitude".
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Equation of Time
Even if your dial includes the longitude correction, its timekeeping will vary throughout the year due to a phenomenon called The Equation of Time. Simply put, the apparent motion of the sun compared to your watch will cause your dial to appear as much as 16 minutes" fast" or "slow" at various times of the year.
The Equation of Time is caused by the combination of two effects: (a) by the earth's elliptical orbit where the laws of Kepler tell the earth to speed faster near the sun than away...that is, at perihelion in December the earth's orbital change is faster that during the June aphelion, and (b) the 23.44° tilt of the earth's axis from the elciptic plane of its orbit.
The chart below shows the amount your dial will vary from clock time through the year. Right click on the graph, select save image for copy, and then print it out and keep it close to your dial. There are dials which compensate for the Equation of Time, but they are complicated.
The Equation of Time can also be created as a table of "fast" and "slow" times. Download it here:
Equation of Time Through the Year.pdf178.58 KB
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