How
to Set a Horizontal Sundial
A
horizontal sundial consists of the dial plate, marked off
in hours, and the gnomon which sits on the noon line and projects
out from the dial plate.
In order to tell the correct local time the gnomon must be
parallel with the earths axis, or, in other words, that it
should point towards the celestial pole. In the northern hemisphere,
this means, for practical purposes, that the gnomon should
point at the Pole Star. One should first check
whether or not the sundial is correctly made for the place
at which it is to be set up. If it is not, the base plate
of the dial must be corrected so that
the gnomon is pointing correctly true north, towards the celestial
pole.
Finding
the direction of true North
Various
methods are suggested in the literature, and are summarised
here, with references to published sources if you need them
- Use
a compass. This is not very accurate, but it will do
for a small garden sundial. Remember that the compass
points to magnetic north, and a correction must be made
for magnetic deviation. (Magnetic deviation at Greenwich
in the UK was 3º58 W, and decreasing by 0º08 annually,
but in some areas of the world it is much higher, and there
are also much more local variations)
- Mark
a shadow at the exact time of local noon The shadow
must be cast by a true vertical object. You can use a plumb
line, a pole aligned vertically with a spirit level, or
a vertical corner of a building. You may need to experiment
to get a good shadow, and to find a reliable method of marking
the shadow at the instant of local noon.
Remember that the sun travels 15º
westwards in one hour, and thus travels 1º westwards
every four minutes. (In the latitude of London, this is
equivalent to 950 feet per second). The time on your watch
must be corrected for this. For example, at Lowestoft
(which is the easternmost point of England at 1º45E,
local noon is exactly 7 minutes earlier than noon in Greenwich.
Penzance in the far West of England is at 5º33W,
and local noon there is 22 minutes and 12 seconds later
than at Greenwich.
Remember too that the sun appears
to be fast or slow compared to watches by an amount discussed
under the Equation of Time The sun is "fast" between 16
April and 14 June, and again between 2 September and Christmas
and "slow" at other times of year. "Fast" means that,
if you are on the standard meridian for your time zone,
the sun will be directly overhead ("sun noon") not at
12:00:00 by your watch, but a few minutes earlier. When
you are setting up your horizontal sundial, you want to
know the time the sun is directly overhead. So you mentally
add the Equation of Time to your watch time, or advance
your watch by the amount of the Equation of Time so that,
at the instant when your watch says 12:00:00, everyone
else's watches will be saying it is actually a few minutes
earlier than that, and it will indeed be "sun noon" The
same thing, of course, applies if you are not on the standard
meridian for your time zone, but you have already taken
account of this with the calculation in the preceding
paragraph.
- Use
the method of equal altitudes This requires a reliably
sunny day, and an accurately level board with a true vertical
nail or stick. In one variation, concentric circles are
drawn around the base of the vertical stick. The position
of the tip of the shadow is noted whenever it just touches
each of the circles in the morning hours and in the evening
hours. If one is lucky there will be two marks on the same
circle. Join them with a line. Bisect this line, and draw
a line from the bisection point to the base of the stick
- this will be a true North- South line.
An alternative is to mark out points on
the track of the tip of the shadow first, and then to
connect them with a line. Then draw a circle to give the
greatest possible distance between the two intersection
points, and as before bisect the line, and draw a line
from the bisection point to the base of the stick - this
will be a true North-South line.
Checking
the angle of the gnomon
Since
horizontal sundials are often mass-produced, they have to be
made for just one latitude. Many are made in Birmingham, where
the latitude is around 52½ deg.N, so the angle between
the gnomon and the dial plate is also 52½ Quite often,
people bring back a sundial when they have been on holiday,
so the angle may be very different. For example, a sundial made
for the south of Spain will have an angle around 37 deg. and
will not tell the correct time if it is set up with the dial
plate horizontal in Southern England where the latitude is 51
deg. Fortunately, this can be compensated for.
First, measure the angle of
the gnomon with a protractor.
Second, you can if you wish
cross-check this measurement and check that the hour lines
have been laid out correctly, by "back-calculating the gnomon
angle from the angles of the hour lines. (The book by Waugh
gives an example of this calculation on p.48, and also a table
showing the correct angles of the hour lines for each degree
of latitude. For example, the angle of the 9am and 3pm hour
lines from the noon line is 26º24 at 30ºN, 29º50
at 35ºN, 32º44 at 40ºN, 35º16 at 45ºN,
37º27 at 50ºN, and 39º20 at 55ºN.)
Compensating for an
incorrect gnomon angle
Third, provide a wedge to bring the gnomon
parallel to the earth's axis. For example, the holiday sundial
brought back from Spain (lat 37ºN) to be set up in Southern
England (lat 51ºN) would have to be wedged up by 14º,
so that the gnomon is at 51º to the horizontal. You can
either measure this angle with a protractor, or you can calculate
the height of the wedge by multiplying the length of the dial
plate by the sine of the correction angle. In this case, the
wedge required for a square sundial with a side of 4 inches
would be 1 inch approx.
Used by permission. Copyright 1997, Sundials
on the Internet, an excellent source for the sundial enthusiast!
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