All sundials record a shadow thrown by the sun onto a scale laid out with markings to represent the time of the day. A sundial is a form of clock. Most commonly the sundial is a small metal plate with a pointer or shadow post to make the shadow on a scale of hours, and it is often mounted on a stone plinth in a garden.
A human sundial or sun clock works in a similar way but instead uses a human being as the post to cast a shadow onto an hourly scale marked on the ground. The person casting the shadow stands on a central stone and puts their arm up to make the shadow.
In this sun clock in Bushey there are two arcs of hour stones arranged around the plinth. The outer arc measures the shadow in Greenwich Mean Time (GMT) hours, and the inner arc measures summer time (BST). Because people use gardens mostly on summer sunny days, this inner arc is used more than the outer arc.
With modern watches, we take the measurement of time for granted, but to early man, the position of the sun in the sky was the main way to tell the time of day. Two times are obvious, dawn when the sun rises, and dusk when it sets. Also, the highest point of the sun in the middle of the day marks out noon, and this is due South.
Because we know that the highest point of the midday sun is South and that the shadow from the sun at this point will point North, then we can find North in the daytime when bright sunlight prevents us from seeing the pole star or indeed any star, although of course they are still there. When we have found this North and South line we can then lay out a scale dividing sunrise and sunset through midday into divisions which we call hours.
In our human sun clock or more correctly an analemmatic sundial, the user stands on the midline of the central plinth stone on the correct month square. If the sun is shining then his or her shadow will point to the time of day just like a traditional sundial on a plinth. The user should stand on the month, and if the body shadow does not reach the hour stone, then raise an arm to make a longer shadow. Remember to raise the arm which is nearest the midline of the standing stone.
The months on the central plinth are marked out to make the user stand on a slightly different place each month of the year. This gives a more precise reading. This sun clock has been set up to be as accurate as possible, but it will never be quite as close as a modern watch – however it will never need a battery or need to be wound up!
There are not many human sun clocks in England, and we believe that this may be the only stone one in Hertfordshire. There are similar sun clocks at Chatsworth House, Blenheim Palace, Longleat and Lincoln Castle. Also there are a number of human sun clocks in school playgrounds, which have been painted onto the ground surface.
Our own sundial has been handcut in Portland Stone by Sarah Stewart-Smith, a stonemason who works in Cornwall. She lived in Bushey some years ago and attended Merry Hill Primary School.
Her work includes stonework and lettering at the Eden Project. She can be contacted through her website at www.stonecarvers.fsnet.co.uk.
The sunclock was given in memory Helena Hurndall (1904 – 2003)
The calculations for setting up this sun clock to the Bushey Latitude and Longitude were provided by Douglas Hunt of Modern Sunclocks at: www.sunclocks.com
The following section is a little about the astronomy and the adjustments that were made to set up this sundial as accurately as possible
All clocks and watches are set to what is called Greenwich Mean Time (GMT) or clock time. This is the time which we use to regulate what we do every day. In the summer we use a time scale which is an hour different called Summer time. Greenwich is the place where the Royal Observatory keeps a ‘mean’ correct time for us all and is accepted as the Standard Time for the British Isles.
This true or Standard Time can be calculated from noon by the sun at Greenwich. Because the earth is spinning under the sun, in Bushey the sun passes exactly overhead one minute thirty-six seconds after Greenwich. We don’t keep this local sun time here, but use the British standard of Greenwich Mean Time, or Summer Time, so our sundial and indeed all our clocks and watches are adjusted to this Standard time.
Before the coming of the Railways and a telegraph system people all over the country kept these different local times, but in 1880 an Act of Parliament made Greenwich Time to be our Standard Time throughout the country. This then meant that railway timetables were all using the same clock time, and also travellers did not have to constantly readjust their watches as they moved around!
In recent years scientifically accurate time is not kept by the sun but by atomic clocks in special laboratories
This sundial has therefore been laid out with a very small position adjustment of the hour stones so that the shadow of the sun at noon reads not local Bushey time, but one minute thirty six seconds early, so that it coincides with the noonday sun at Greenwich. In practice this amount is so small that it would be difficult to read the shadow of a sundial with this accuracy, but if we set up a similar sundial in Penzance, then it would have to be set up to read over twenty – two minutes early to accurately coincide with our watches, and if this difference were not built into the sundial arrangement it would be very noticeable.
We think of our earth going around the sun in a circle, but in fact it moves in a pathway which is a slightly lengthened oval, called an ellipse. Because of this the earth actually speeds up and slows down a small amount twice during the course of a year. This change of speed and other small variations cause the shadow thrown by the sun to also change its position slightly during the year. This variation was calculated by our first Astronomer Royal, John Flamsteed in 1672. The correction applied to the sun’s shadow so that the shadow scale corresponds to our clock time is called the ‘equation of time’ and is repeated exactly the same year after year.
This alteration can be made by looking up the adjustment in tables or on a graph*. It but can also be made by moving the gnomon or shadow post to make the sun’s shadow fall in a slightly different place. This is difficult in most simple sundials, which have no easy way of adjusting this change of the shadow position. Analemmatic sundials like this one in Bushey have the unique ability to make this change by moving the person making the shadow.
To do this for each day of the year would be too confusing, so instead the correction is made in monthly amounts by using the month markings on the central date stone. You can see that some of these months occupy a larger space, indicating that a larger adjustment is needed at those times.
We tend to forget that the sun is not moving across the sky from East to West, but is actually relatively still. It is the earth that is turning from West to East which allows us to be illuminated by the sun during the day; and when it turns further we don’t get any sunlight and it is dark until it rotates once more into the sunlight the next day. The earth turns on its axis once every twenty-four hours and by happy coincidence an imaginary extension of this axis into space meets the pole star, so we can always find true North at night – if we can find this star. In the daytime we know that the sun is due south at mid-day and during our adjusted summer time it is due south at one o’clock.
Sundials were once an important part of life, playing an essential role in marking the time of day. In modern Britain they are more of historical interest, but understanding their construction and use provides a basis to practical map reading, navigation and to some knowledge of astronomy.
Photograph and drawing by Nick Overhead
* This graph has a curved shape called an ‘analemmatic curve’ hence the name for sundials using this method to improve the accuracy of time measurement.