Sundial Accuracy

A properly designed and installed sundial can be a very accurate means of telling time, down to intervals of less than a minute.

I won't go into mathematics, but on a sundial 16 inches (40cm) in diameter, shadow of gnomon will move about 1/30th of an inch, or just under 1mm, in a minute. This may be small, it's enough for our eyes to see.

Two Major Problems

Apart from frequent absence of sunlight (Problem 1), all sundials show time by cakibrating outwards from position of sun at noon, and if you live east or west of me, your noon is different to mine.

Although earth moves around sun, we see it other way. The sun appears to move from east to west across sky, and local noon is when it's vertically overhead. But if you live 100 miles west of me, my noon is still your late morning, and your noon is my early afternoon. This would be inconvenient if we used our sundials to arrange a lunch date, but a real problem if I had a plane to catch in another city.

Solar Time and Official Time

People managed to live with this problem until communications and transport became faster. Imagine calculating train timetables when Boston, New York and Buffalo all worked on different local times.

The answer was development of local time zones. US Railways did this in 1883, but in 1914 world's governments agreed to divide globe into 24 zones, each 15 degrees of longitude in width, and each one hour different in time to its neighbours. Boundaries were altered slightly to account for state and national borders.

There are four time zones in contiguous 48 states of USA: Eastern, centred on 75 degrees W longitude; Central, on 90 degrees; Mountain, on 105 degrees; and Pacific, on 120 degrees. Noon was identified astronomically for each of these meridians (now it's done by atomic clocks), and accepted everywhere else in zone.

Noon on sundials in places very close to these longitudes will correspond to official noon. For every degree east or west of central meridian, for 7.5 degrees either side, you will need to add or subtract four minutes respectively to correct your sundial.

A few other adjustments are necessary to compensate for irregularities in earth's path around sun - not too difficult to make but theory is beyond this article.

They add to inconvenience, and that's why sundials have been superceded by more convenient and reliable forms of time keeping. But problems with time zones and orbital paths can be corrected, and there's no reason why you can't find correct time from your sundial.

No reason, that is, provided it has been properly installed in your garden. And that's subject of another article.

Finding North

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

One way, suitable for northern hemisphere, is to identify pole star. This is very close to projected position of earth's axis, about which sun and stars seem to revolve. You could mark direction from your sundial's location to pole star, but this method isn't quite accurate, and needs to be done in dark. And 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, needle on a compass points to magnetic north, not true north, which is what we want. The difference between two is called magnetic declination, and is usually shown on good topographic maps. And while a simple addition or subtraction of difference between two norths should give you right direction, there may be some local magnetic effects which can't be compensated for.

The third method goes back to ancients - and there were some pretty smart operators around in 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 stick up vertically at location you have chosen for your sundial, so that top of its shadow falls on sheet of paper or board. If you stand with your back to sun, behind pole, set paper up so that morning shadow falls on its left hand side.

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

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

You can find true north in other ways - again I suggest you try google as suggested above.

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