Finding the time of local noon
Since this page was written, we have
developed a dedicated website, www.solar-noon.com
which, if you key in your latititude and W, e have left this
page in for people who prefer to work through the theory rather
than just letting a computer do the work! Either way, you will
end up with a good value for solar noon, and, if you orient
your sundial so that it reads 12 o'clock at this time, it will
be accurately oriented to true North |
Sundials measure time as it is. Watches measure time as we would
like it to be, because it is both easier and more convenient.
Sundials are based on the idea of measuring time before and after
noon. Noon is when the sun is at its highest in the sky. People
tend to assume that the time between noon one day and noon the next
is exactly 24 hours.In fact, the length of a day varies slightly
throughout the year. The shortest days are some 23 hours, 59 minutes
and 40 seconds long and occur around 15 September, while the longest
days around Christmas are some 24 hours and 30 seconds long. The
reasons for these variations are complex, and are explained in details
in our companion website, Sundials
on the Internet, on the Equation
of Time page).
Watches are based on the convenient assumption that all days in
the year are exactly 24 hours long. Sundials take the days as they
are, varying in length from 24 hours and 30 seconds on December
22 and 23 hours 59 minutes and 40 seconds on 15 September as stated
above.
1. Find out the longitude of the place where the sundial will
be installed You can do this from any good atlas - longitude
lines run from north to south on the atlas. Many atlases give the
latitude and longitude against the place names in the index. The
latitude and longitude obtained in this way will not be the exact
latitude and longitude of your sundial, although it will usually
be good enough for these purposes unless you live in a large city.
For a more exact method, please refer to our page on Finding
your Latitude and Longitude
2. Find out the longitude of your standard meridian The
world is divided into time zones 15 deg. apart, measured from longitude
0 in Greenwich, England. Thus, the standard meridian for the United
Kingdom, Ireland and Portugal is the prime meridian of 0 deg, while
the rest of continental Europe keeps European Time, for which the
standard meridian is 15 deg. E of Greenwich (which passes through
Prague, the capital of the Czech Republic). The time zones of North
America are:
Zone |
Prime meridian
deg. W |
City near that
longitude |
Hours earlier
than Greenwich |
Atlantic |
60°
|
Glace Bay NS |
4
|
Eastern |
75°
|
Philadelphia PA |
5
|
Central |
90°
|
Memphis TN |
6
|
Mountain |
105°
|
Santa Fe, NM |
7
|
Pacific |
120°
|
Fresno CA |
8
|
3. Calculate the difference between your longitude and the prime
meridian (e.g 4 deg 10 secs West for Plymouth), Subtract the
longitude of the prime meridian of your time zone (eg 0 for Greenwich).
Since the sun takes 1 hour to traverse 15 degrees, the sun crosses
the longitude of Plymouth later than it is at Greenwich. It will
be 4 minutes later for each degree of longitude, and 4 seconds later
for each minute of longitude. So 4 deg 10 secs means that solar
noon in Plymouth will be 16 minutes and 40 seconds later than it
is at Greenwich.
This example and three others are given as worked examples below.
+ indicates that the place is West of its prime meridian (and thus
the sun is overhead later than it is at the prime meridian); - indicates
that the place is east of its prime meridian, and thus the sun is
overhead earlier than it is at the prime meridian)
Place |
Plymouth, England |
Barcelona, Spain |
New Harbor, Maine |
Boise, Idaho |
Long. of place |
4° 10' W |
2° 9.5' E |
69° 30. W |
116° 12.8' W |
Long. of prime meridian |
0° . |
15° E |
75° W |
105° W |
Difference |
+4° 10' |
-12° 51.5' |
- 5° 30' |
+11° 37.2' |
Time equivalent |
+16 mins 40 sec |
-51 mins 26 sec |
-22 mins 0 sec |
+46 mins 29 sec |
4. Look up the time of solar noon at your prime meridian
in the table below
Time of solar noon at the prime meridian
|
1st |
Corr./day |
11th |
Corr./day |
21st |
Corr./day |
|
hh:mm:ss |
secs. |
hh:mm:ss |
secs. |
hh:mm:ss |
secs. |
January |
12:03:09 |
+20.5 |
12:07:38 |
+21 |
12:11:05 |
+12 |
February |
12:13:33 |
+4.5 |
12:14:19 |
+3 |
12:13:49 |
-9.5 |
March |
12:12:34 |
-13.5 |
12:10:18 |
-17 |
12:07:28 |
-20 |
April |
12:04:08 |
-20 |
12:01:16 |
-13.5 |
11:59:00 |
-11 |
May |
11:57:09 |
-5 |
12:56:20 |
+6 |
12:56:26 |
+7 |
June |
11:57:42 |
+9.5 |
11:59:21 |
+12.5 |
12:01:28 |
+12.5 |
July |
12:03:33 |
+10.5 |
12:05:16 |
+6 |
12:06:15 |
0 |
August |
12:06 16 |
-6 |
12:05:14 |
-12 |
12:03:16 |
-17 |
September |
12:00:12 |
-20 |
11:56:52 |
-21 |
11:53:20 |
-21 |
October |
11:49:55 |
-17.5 |
11:46:58 |
-13 |
11:44:45 |
-6 |
November |
11:43:40 |
+2 |
11:44:00 |
+12.5 |
11:45:44 |
+18m5 |
December |
11:48:46 |
+25 |
11:52:58 |
+29 |
11:57:44 |
+30 |
5. Calculate the time of solar noon at your location (if
you think it will be sunny that day at the time of solar noon).
For example, on 11th March, solar noon is at 12:10:18 at the prime
meridian, so solar noon at Plymouth is 12:10:18 plus 16:40 which
gives 12:28:50 by your watch. Similarly, solar noon at Barcelona
is 12:10:18 - 51:26 which gives 11:19:52 on your watch. The calculation
is tabulated below..
Place |
Plymouth, England |
Barcelona, Spain |
New Harbor, Maine |
Boise, Idaho |
Time of solar noon at prime meridian |
1210:18 |
1210:18 |
1210:18 |
1210:18 |
Time equivalent of longitude (see above) |
+16 mins 40 sec |
-51 mins 26 sec |
-22 mins 0 sec |
+46 mins 29 sec |
Time by your watch of solar noon at this
location |
12:26:58 |
11:18:52 |
11:48:18 |
12:56:45 |
See note on daylight saving time below |
Sundial apparently fast or slow by your
watch |
26 mins 58 secs slow |
41 mins 8 secs fast |
11 mins 42 secs fast |
56 mins 45 secs slow |
In the last line of the table above, the times are noted as "apparently"
fast or slow because it is the watch which is keeping a purely artificial
time, and the sun which is continuing in its course as it has done
for millennia. As a practical day-to-day matter, however, it may
be helpful to think of the sundial being, say, just about 27 mins.
slow in Plymouth on 11th March. Then we know that at 9:27 by our
watch, the sundial will be reading 9 am exactly, at 10:27 by our
watch it will read 10 am exactly, and so on.
5. If your country operates Daylight Saving Time or Summer
Time when all clocks are altered to read an hour ahead of the
standard time, add 1 hour to your calculated time of solar noon.
For example, on 11th August, solar noon occurs at 12:03:13.at the
prime meridian, so solar noon at Plymouth is 12:03:13 plus 16:40
plus 1 hour which gives 1:19:53 on your watch. Similarly, solar
noon at Barcelona is 12:03:13 - 51:26 plus 1 hour which gives 12:11:47
on your watch
6. Set your watch accurately by a radio time signal.
7. Set up your Spot-On Sundial. At the exact time of solar
noon which you have calculated, rotate the sundial on the baseplate
until the ray of sunlight falling between through the slot in the
gnomon is exactly over the dotted noon line.
Note that the Spot-On Sundial has the hours marked in two ways.
Roman numerals indicate the winter hours, and Arabic numerals indicate
Daylight Saving Time.
You may find it more convenient to set the Sundial at some other
time than solar noon. It is nearly as accurate to set the shadow
when it is exactly on one of the hour lines, using the number of
minutes apparently fast or slow from your calculations to know the
corresponding time on your watch.
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