The Perfect Garden Sundial

Is it possible for a garden sundial to tell perfect
time?

The chances are very good if you can make one yourself, or
alternatives will take either time or money.

The reason for this is that apart from adjustments
for time zones and daylight saving , covered in another
article, there are three things that must be done.

Compensating for the Earth's Path Around the Sun

If the earth followed a perfect circle as it revolved around
the sun, life with sundials would be easy. But its path is
elliptical or oval, and this causes errors of up to 16 minutes
in sundial time at some times of the year. The corrections
are straightforward, and can be made using a table, or from
a figure 8 line called the Analemma, often seen on
old globes and sundials.

Correcting for Latitude

Let's consider the components of a sundial for a moment.
It consists of a dial on which the time divisions, and
sometimes other information is marked, and a triangular
piece which sits vertically on the dial. It is called the
gnomon, pronounced nomon, and the part which casts
the shadow onto the dial is called the style.

type "how + sundial", without the quotes but with the +,
in the search box.

To be accurate, the angle between the triangular part of the
gnomon and the horizontal must be the same as the latitude
of the place it is to be placed in (You can find the latitude
- and longitude - of your home from any topographic map or
good atlas).The arrangement and distance between the
hour markings on the dial must also be correct for the
latitude.

Hmmmm! This means that unless you are very lucky, that
elegant sundial in your local garden supply shop will probably
not show the time particularly well. It may be calibrated
for an average latitude (commonly 45 degrees), which is good
if your latitude is not too different. Or it may be purely
ornamental and will really only be useful around noon.

Now of course this doesn't matter at all if you are looking
for something pleasing to the eye, and don't mind answering
the inevitable question "Does it tell the time?" But if
you'd like your sundial to be more useful, make sure you
find out which latitude it is calibrated to.

Once you know this, all you need to do to compensate is to work out the difference, and tilt the dial towards or away from due south depending on whether you need to add to or subtract from the latitude the sundial was designed for. There may be slight differences to the ideal spacing of the hour marks, but the apparent time will be reasonably close.

Finding North

The final essential in sundial installation is to make sure the
gnomon is oriented north-south. Sounds easy and, with a little
patience, it is.

One way, suitable for the northern hemisphere, is to identify
the pole star. This is very close to the projected position
of the earth's axis, about which the sun and stars seem to
revolve. You could mark the direction from your sundial's
location to the pole star, but this method isn't quite
accurate, and needs to be done in the dark. And the
southern hemisphere doesn't have a pole star.

Method 2 uses a compass. Sounds pretty simple, doesn't it.
But you guessed it - there are some complications.

Firstly, the needle on a compass points to magnetic north,
not true north, which is what we want. The difference between
the two is called the magnetic declination, and is usually
shown on good topographic maps. And while a simple addition
or subtraction of the difference between the two norths
should give you the right direction, there may be some
local magnetic effects which can't be compensated for.

The third method goes back to the ancients - and there were
some pretty smart operators around in the old days.

You'll need a stick, some paper or board, a marker, a tape
measure or long rule, a sunny day, and a bit of time on your
hands. Set the stick up vertically at the location you have
chosen for your sundial, so that the top of its shadow falls
on the sheet of paper or board. If you stand with your back
to the sun, behind the pole, set the paper up so that the
morning shadow falls on its left hand side.

Now mark the end of the shadow with a permanent marker. Come
back through the day and mark the new positions of the tip of
the shadow - the more often the better. As the day goes on,
you'll notice the marks form a curve.

Later in the afternoon - any time after three is OK - connect
the marks you've made into the smoothest curve you can manage.
Do this while the pole and paper are still in place. Then
carefully measure the distance between the base of the pole
and the curve. The shortest distance corresponds to true
north. Mark it in some way, and align the gnomon in the same
direction when you put your sundial in place.

You can find true north in other ways - again I suggest you

Once you have set up your sundial, check the time, compensate
for differences with your official time zone, pat yourself
on the back, and if the sundial tells you it's after midday,
pour a glass of your favourite beverage and put your feet up.