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Mean Solar Time
© Copyright 2000, Jim Loy
Solar Time is the time as measured by the sun. It is not uniform; it speeds
up and slows down, because the earth moves slower and faster in its orbit
around the sun, and because of different distances from the sun. Our clocks use
Mean Solar Time, which is uniform, being averaged out over a year
(365.243... days). So the sun is not where the clock says it is. It may be as
much as 16 minutes (of time) off. So if you are concerned about sunrise or
sunset or eclipses, then you must adjust the position of the sun somewhat. How?
On a globe of the earth, there is often a figure 8 shaped scale, called an
analemma, in the Pacific Ocean somewhere. It should list some days and months,
and give an indication of how many minutes to add or subtract to your sundial,
in order to get mean solar time. I haven't looked at an analemma for many years
(none of my encyclopedias or astronomy books show one); and I don't remember
just how it is used. Here is a table that does much the same thing (giving
minutes and nearest 10 seconds):
| Jan. 1 |
-3:20 |
| Jan. 15 |
-9:20 |
| Feb. 1 |
-13:30 |
| Feb 15 |
-14:10 |
| Mar. 1 |
-12:20 |
| Mar. 15 |
-8:50 |
| Apr. 1 |
-3:50 |
| Apr. 15 |
+0:00 |
| May 1 |
+3:00 |
| May 15 |
+3:40 |
| June 1 |
+2:10 |
| June 15 |
-0:30 |
| July 1 |
-3:50 |
| July 15 |
-6:00 |
| Aug. 1 |
-6:20 |
| Aug. 15 |
-4:20 |
| Sep. 1 |
+0:00 |
| Sep. 15 |
+4:50 |
| Oct. 1 |
+10:30 |
| Oct. 15 |
+14:20 |
| Nov. 1 |
+16:20 |
| Nov. 15 |
+15:20 |
| Dec. 1 |
+10:50 |
| Dec. 15 |
+4:50 |
This is known as The Equation of Time. The word "equation"
here apparently does not refer to an algebraic equation, but the method of
equalizing (making uniform) time. Add these numbers to solar time, to get mean
solar time, or subtract to do the reverse. The times vary a few seconds, from
year to year, as the calendar is not synchronized with the 365.243... day year.
Addendum:
Here is my drawing of an analemma, roughly as it appears on some
globes. I have only shown a few of the days. Normally the analemma has many of
the 365 days marked. It extends from the Tropic of Cancer in the north to the
Tropic of Capricorn in the south. The analemma not only tells how much the sun
appears to be ahead or behind clock time (in degrees on the globe), but it also
shows how far north or south the sun appears to be. You can figure out how many
minutes the sun is ahead or behind, by converting from degrees on a globe to
hours and minutes on a clock (24 hours=360 degrees).
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