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This section deals with a few important differences between
sun time and the normal time that your watch or clock displays. If you
want the best accuracy in using the sun for simple navigation or your
want a better sun dial these differences need to be taken into account.
The simplest and potentially largest error is not a natural
difference but one created by politicians. This is day light saving time
or summer time which was first suggested by Benjamin Franklin in an essay
published in 1784. The idea was revived by William Willett, in the United
Kingdom, in 1907. Parliament rejected the idea in 1908 but it was re-introduced
and accepted in 1916. It involves advancing clock time by an hour during
the summer months. In the United Kingdom this happens from the last Sunday
in March to the last Sunday in October. To correct for day light saving
time subtract one hour from the time on your watch. This means that 12
noon sun time occurs at 1 pm watch time. The sun moves 15° in an
hour so forgetting this correction will result in large errors. If you
are not in the United Kingdom check you local arrangements to see if
daylight saving time is used and whether the change is one hour or an
alternative value.
The next error is called the equation of time and is caused
by the sun not keeping very good time. Sun time varies throughout the
year because the Earth is tilted on its axis and the Earth’s orbit
round the sun is elliptical rather than circular. The important thing
to remember is that the error can be up to 16 minutes which is a significant
error. The table below gives the approximate corrections in minutes that
need to be made to convert sun time to clock time, positive values are
added to sun time and negative values are subtracted. Reverse the sign
of the corrections to convert clock time to sun time. For dates between
those listed use the correction for the nearest listed date, this will
usually give an error of less than 0.5°. Alternatively you could
estimate a value between the figures given for the dates each side of
the one you want to correct, this refinement is not usually required.
There are four dates when the error is zero, these are worth memorising.
Sun Time Corrections (Minutes)
| |
Day |
| Month |
1 |
6 |
11 |
16 |
21 |
26 |
31 |
| JAN |
3 |
+5.5 |
+7.5 |
+9.5 |
+11 |
+12.5 |
+13 |
| FEB |
+13.5 |
+14 |
+14 |
+14 |
+13.5 |
+13 |
|
| MAR |
+12.5 |
+11.5 |
+10 |
+8.5 |
+7.5 |
+5.5 |
+4.5 |
| APR |
+4 |
+2.5 |
+1 |
0 |
-1 |
-2 |
|
| MAY |
-2.5 |
-3 |
-3.5 |
-3.5 |
-3 |
-3 |
-2.5 |
| JUN |
-2.5 |
-1.5 |
-0.5 |
0 |
+1.5 |
+2.5 |
|
| JUL |
+3.5 |
+4.5 |
+3.5 |
+4.5 |
+5.5 |
+6.5 |
+6 |
| AUG |
+6 |
+5.5 |
+5 |
+4 |
+3 |
+1.5 |
+0.5 |
| SEP |
0 |
-1.5 |
-3 |
-4.5 |
-6.5 |
-8.5 |
|
| OCT |
-10 |
-11.5 |
-13 |
-14 |
-15 |
-15 |
-16 |
| NOV |
-16 |
-16 |
-16 |
-15 |
-14 |
-12.5 |
|
| DEC |
-11 |
-9 |
-7 |
-4.5 |
-2 |
0 |
+2.5 |
The third error is due to the difference in longitude between
your location and the longitude that is used as the time reference for
your time zone. The world is divided into 24 time zones, each 15° of
longitude wide. The time for each zone is set to mean sun time at the
centre longitude of the zone. Sun time varies with longitude at a rate
of 4 minutes per degree. To correct sun time to clock time multiply the
difference between your longitude and the centre longitude by 4 and add
the value , in minutes, to the sun time if you are west of the centre
longitude and subtract it if you are east of the centre longitude. Reverse
the addition or subtraction to convert clock time to sun time. If you
are not in the United Kingdom check the details of your local time zone,
some of them are not 15° wide. My time zone map shows Australia as
having very different time zones, if this is still correct you need to
take care if you are in Australia!
All of the corrections given need to be combined to arrive
at a corrected sun time or clock time. Take care with the sign of the
various corrections.
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