Correcting for your position in your Time Zone
When solar time became inadequate for railway timekeeping, a system of time zones was initiated creating 24 time zones, each 15° wide - within which all clocks would tell the same time. (Basic arithmetic: The earth's 360° circumference was divided into 24 one hour segments, and the result was 24 time zones each 15° wide.)
However, the sun moves across each artificial time zone at its own pace.
If you wish your sundial to compensate for its position in its 15° time zone and approach Standard (clock) Time, you will need to apply a longitude correction.
This correction is determined by the relationship between the longitude of your sundial and the longitude of the central meridian of your time zone. The central meridians of all 24 time zones can be found in the table below.
If you are setting up a commercial sundial, the longitude correction will not be built into the dial, but it will be a constant amount which you must apply at each reading. If your longitude is west of your central meridian, add four minutes to the time on the dial for each degree. If you are east of your central meridian, subtract. This correction can be built into your dial if you are making one.
Even after the longitude correction is applied, your dial will not consistently tell Standard Time due to The Equation of Time which varies throughout the year. See below.
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