Finding the time of local noon
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 3 0 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 in late December
and 23 hours 59 minutes and 40 seconds on 15 September as stated
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
|City near that
|Glace Bay NS
|Santa Fe, NM
3. Use the Spot-On Sundials Solar
Noon Calculator. This will print out a table showing the
exact time of Solar Noon for each day of the year, which you will
need to set up your Spot-On sundial accurately. (It will also be
helpful to keep it for later, so that you can follow the variations
in the Equation of Time throughout the year. (It will also help
you when people look at their watch and say that the sundial is
"wrong"; you can explain that in fact sundials and watches are measuring
slightly different concepts of time, and both do the job they are
supposed to do very well.) If you prefer to do the calculations
for yourself, please refer to our page on finding the time of local
6. Set your watch accurately by a radio time signal.
7. Set up your Spot-On Sundial. At the exact time of solar
noon shown in the table for the day your are setting up the Spot-On
sundial, 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.