* Read our previous article on the history of sundials
Following on from last week’s article on the history of sundials, this week we tell you how to install and position a horizontal sundial correctly.
Sundials are scientific instruments which can tell the time with precision if they are well-made and set up accurately. This is relatively simple as long as you pay attention to the finer details.
Your sundial needs to be placed on a plinth which is horizontal, preferably in an area that sees the sun and it must be oriented towards the true North, which is very close to the Pole Star.
Choose your location carefully. If your site has trees or buildings near to it you should try to organise the sundial so that the view to the West is as open as possible, because most people will be inspecting the sundial in the late afternoon.
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If you have trees or buildings to the South that will not be so important as the sun will not be so high in the sky at the middle of the day.
There are 4 ways to find due North:-
1) Traditional – using a pole in ground
2) A Compass
3) The website to find solar north
4) Or the new sundial app.
1. The traditional way of finding true North is to put a pole in the ground, make sure it is accurately vertical with a plumb line or spirit level on two sides at right angles, and then draw circles on the ground from the base of the pole. On a sunny day, note the position where the shadow of the tip of the pole just touches one of the circles in the morning, and again in the afternoon. Bisect this angle, and that will give you an accurate North-South or meridian line.
2. Another way is to use a compass, but this is not recommended because the length of the needle is very short, and anyway it points to magnetic North, and many people find the correction to true North difficult to apply.
3. Modern technology has given us two new ways to find true North. If you know the exact time of solar noon, and you rotate your sundial till the shadow falls on 12, it will be pointing to true North. There is a very useful website, www.solar-noon.com, which will tell you the exact time of solar noon wherever you are in the world. The picture shows the printout for the first few days of each month for Windsor Castle.
4. An innovation this year is SNAP – the sundial app which shows you, for several thousand cities worldwide, the sun time and clock time at that location, with a picture of a sundial face and the shadow on it. If you can find a city on the same longitude as you are, you can set your sundial to the sun time shown whatever time of day it is. And if the longitude is not quite right for you, you can just add 4 minutes for every whole degree, or 4 seconds for each second of longitude, if you are due west of your chosen city, or subtract if you are due east of it.
Accuracy depends on the way they’re made. It is possible to read a scientific sundial to within a couple of minutes, although some sundials will be less accurate.
What do you do when the clocks change? Nothing. The position of true North does not alter just because of an act of Parliament! You adjust your mind, not the sundial. And just remember to add one hour in the summer.
Piers Nicholson runs a leading sundial information website www.sundials.co.uk and has also designed an innovative range of spot-on sundials in brass and stainless steel; the range includes horizontal, polar, and equatorial sundials.
This is an article from ProjectBook which provides a wide range of information for the conservation, restoration, care and repair of period and listed buildings. spot-on sundials are members of ProjectBook’s Products Directory which provides a showcase for specialist products, suitable for use on traditional properties. Updated daily with new content, the website features the Heritage Register, a products directory, informative articles, current news, events and more. For more information, visit www.projectbook.co.uk.
SNAP – the sundial app