BC Gnomonics
From Local Apparent Time
to Civil Time
Because of the eccentricity of the earth's orbit, as well as the tilt of its axis, the apparent movement of the sun over the course of the year is not uniform. Mean solar time (MST) is based on the movement of a more predictable, fictional sun. To bring local apparent time (LAT) (the time shown on sundials) into accordance with the time shown on mechanical and digital clocks, certain corrections must be applied.
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Steps 1-5 explain how to determine your longitude correction. This is an unchanging value. Effectively, correcting for longitude gives you local apparent time (sundial time) for the standard meridian of your time zone.
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Step 6 explains how to utilize the equation of time. This is a universal value that varies over the course of the year. By applying the equation of time, you transform local apparent time into mean solar time.
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With both the equation of time and the longitude correction, you have mean solar time for the standard meridian of your time zone.
If applicable, add an hour for daylight saving time.
You now have civil time, the time read on clocks.
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Step 7 is optional and explains how to combine the longitude correction, equation of time, and DST into one single correction.
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1. Take note of your time zone of residence and the meridian (longitude) upon which it is based.
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Time zones of the continental United States:
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Pacific Time: 120ºW
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Mountain Time: 105ºW
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Central Time: 90ºW
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Eastern Time: 75ºW
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(Click to enlarge)
2. Take note of your own longitude. This can easily be found online.
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3. Find the difference in degrees between your longitude and the standard meridian of your time zone.
For example, this difference for someone living on the 70th meridian would be 5º, seeing as though the standard meridian of their time zone would be 75ºW.
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4. Multiply the difference between your longitude and that of the center of your time zone by 4.
If you live to the West of the center of your time zone, make this number positive.
If you live to the East of the center of your time zone, make this number negative.
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5. The resulting value is your longitude correction in minutes. When reading a sundial that has not already been corrected for longitude, add or subtract this number from the time displayed on the dial.
Remember, this value is unique to your location and must be recalculated for all longitudes. For a given location, however, this value does not change.
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6. The equation of time yields a number from -16 to +14, depending on the time of year. There are a number of resources with which to calculate this value for any given day. Today's value can be seen here.
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Reading the equation of time:
The EoT value for any given day can also be gleaned from this graph.
To use it, trace a line up from the current date and see which horizontal mark intersects with the red line.
If the date reaches the red line above the 0 mark, then the value is added.
If the date reaches the red line below the 0 mark, then the value is subtracted.
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7. If you find it more convenient, the values from the equation of time, longitude correction, and daylight saving time (if applicable) can be combined into a single correction.
To do this, consider each of the three values as either a positive or negative number of minutes.
For example, if you live to the east of the center of your time zone, then the value is negative. If west, it is positive.
Next, add or subtract from the longitude correction the value from the equation of time. If above the 0 mark, the value is negative. If below, it is positive.
These two steps will yield you either a negative or positive number. If daylight saving time is in effect, add 60 minutes to the total.
The final value is the total "master" correction for the day.
Because of the variation of the EoT, this value will drift slightly as days pass. It is recommended to recalculate this master correction every few days.