Sundial design
- the work of Piers Nicholson
The design process for the Tylers
and Bricklayers sundials
I have often been asked how the ideas
for the Tylers and Bricklayers sundials evolved, since many people are interested
in the actual process of design work. The essence of it, I think, is that you
never know at the beginning where the design will end up, and that it will always
take a lot more time than you thought! So, for what it is worth, I have set out
as closely as I can remember it the early stages of the design.
In late 1996, I was invited to serve on the Millennium Committee of the Worshipful
Company of Tylers and Bricklayers, which is one of the ancient livery companies
of the city of London in England. The committee was charged with selecting a suitable
project or projects highlighting the Company's interests that they should support
to mark Millennium year.
Having a long-standing personal interest in sundials, I was naturally keen that
the Company should be involved with a sundial, but it was a bit difficult to see
how his could sensibly be done in a way that would also be an interesting piece
of brickwork. My initial suggestion was that the Company should consider something
along the lines of the Great Sundial at Jaipur in India, but such a massive structure
was unfortunately considered impractical on the grounds of cost and space!
The design of the sundial went through a number of stages, drawing together the
constituent elements of gnomon, dial plate and plinth to best represent the Millennium,
and the Tylers and Bricklayers in a working sundial. Nearly all sundials have
a gnomon (which casts the shadow) pointing to the Celestial Pole. This means that
at the North Pole, the gnomon would point straight up, while at the Equator, the
gnomon would be horizontal. Between the two, the gnomon slopes upwards by an angle
equal to the latitude.
Different sundial types are distinguished by the plane of the dial plate, which
has the hour lines marked on it. The most common type is the horizontal sundial,
where the dial plate is horizontal, usually of metal, and mounted on a pedestal
to make it easier to read and less prone to accidental damage. The pedestals are
usually made of stone, and there is no particular reason for substituting this
with brick. The next most common type is a vertical sundial, typically mounted
on a wall of a building such as a church; this obviously gives no scope for brickwork
as part of the dial. The design presented to the committee was for a polar dial,
in which the dial plate lies in the plane of the celestial pole, and is thus parallel
to the gnomon. The first version considered for this design had a plinth of brick,
sloping at 38.5 degrees to the horizontal, so that the top surface of the plinth
was at the same angle as the latitude of 51.5 degrees. The gnomon was made from
a brick raised out from the surface of the dial plate, and the dial plate itself
was made from tiles with the hour numerals marked on each tile. This design turned
out to have considerable practical difficulties: the tiles would have to be specially
made to exact dimensions, and some permanent method found of putting in the hour
numerals. Ideally, each tile would have had slanting edges so that gradual wear
over the years would leave the mortar joint marking the hour in the same angular
position from the gnomon.
The gnomon assembly had to be at a reasonable height if it was to be read easily,
and it had to be sufficiently wide to accommodate the brick gnomon. There was
an advantage in having it quite wide, since it would show more hours of the day.
One advantage of brick is that it is a very durable material, so that a sundial
made of brick should, barring accidents, have a very long life expectancy. It
was at this stage that the design acquired the name of the "Sundial for the Third
Millennium". A calculation was done of the number of bricks required to make the
plinth, and it was found that, with a little juggling of the dimensions, it could
be made with exactly 2000 bricks! It seemed entirely appropriate that a sundial
for the third millennium should have one brick for each year of the first two
millennia.
A wooden model was built in Epsom in order to test out the visibility of the shadow
at a distance from the gnomon. Because the sun has an angular diameter, the shadow
gets somewhat fuzzy at a distance from the gnomon. The small area between shadow
and full sunlight is known as the penumbra, the part that is illuminated by one
part of the sun but is in the shadow from the remainder of the sun. The area of
penumbra on the polar dial plate is larger in the early morning and the late afternoon
when the shadow is furthest from the gnomon. It had originally been hoped that
it would be practical to have the width of the sundial at around 1800 mm (6 ft).
Tests showed that the shadow was not sufficiently clearly defined at that width,
so it was decided to limit the width of the dial plate to 1575 mm (5 ft 2 ins)
These experiments also showed that it was possible to have lines indicating every
ten minutes rather than every hour (which had been a feature of the brick-and-tile
design). It was decided to make the gnomon and dial plate out of stainless steel,
which would be very strong, resistant to damage, and could take accurately engraved
lines at close intervals. An added bonus from the use of stainless steel was that
it permitted the ends of the dial plate to be bent up parallel to the gnomon,
so that the dial scales could be read for the full 12 hours. (Polar dials, with
their dial plates parallel to the earth's axis, present their front face to the
sun for 12 hours from 6am to 6pm. If a dial plate of this width is flat, the shadow
is too fuzzy to read for most of the first and last hour of the twelve).
There was a further decision to be made about the length of the gnomon edge and
the height of the dial plate. If both these dimensions were the same, the shadow
would only cover the dial plate completely at the spring and autumn equinox. In
the summer, when the sun is higher in the sky, the shadow would fall only on the
lower part of the dial plate, and in the winter only on the upper part. It was
therefore decided to extend the width of the gnomon into an "oxhead" shape, so
that the shadow edge of the gnomon is substantially longer than the width of the
dial plate, thus ensuring that the shadow falls across the whole of the dial plate
at all times of year.
The design of the sundial was now finalised, and the long drawn-out process of
asking the City of London Corporation whether they would accept such a gift, and
seeking the necessary site and approvals began.
The siting of a sundial is obviously critical, if it is to show the correct time.
It is less obvious that the most critical parts of the arc are roughly from east
to south-east, and west to south-west, since it is in this sector that the sun
rises and sets in the winter, and it is therefore desirable to have a clear view
of the horizon. The sector to the north of this is almost completely unimportant
for a polar dial, since the sun will not be shining on the dialplate before 6
in the morning or after 6 at night. The sector to the south is of intermediate
importance-it is not necessary to have a view of the horizon in this sector, but
any tall obstructions nearby will obscure the sundial at some times of year.
The implication of this is that the best sites are likely to be on the north bank
of the river. The City of London has a number of such sites. The first one we
suggested -at Old Billingsgate-was considered unsuitable by the City Engineers,
but they kindly suggested two others. One of these, outside the City of London
School for Boys on the riverside walkway near Blackfriars Bridge, is technically
an excellent site, and has the added merit that it gets a high volume of foot
traffic; we could not have asked for a better site.
This was the finish of the design process, but it was really only the beginning
of the story. It is one thing to think of these things in the mind and to build
a model, but it is quite another to ensure that it is actually possible to build,
and in such a way to ensure a very long life. Fortunately, it was at this stage
that the Royal Engineers became involved, and did all the detailed design of the
brickwork and the gnomon, and the project management of the building work. This
involved many different people, listed in the main leaflet, contributing their
individual skills to the success of the enterprise.
The involvement of the Royal Engineers resulted in another sundial to the same
design being built outside their Museum in the Brompton Barracks at Chatham. The
Tylers and Bricklayers Company also had the opportunity to present a third sundial
of this design to English Partnerships, who are redeloping the Greenwich Peninsula.
This one is located midway between the Dome and the Thames Barrier, on the east
side of the peninsula near the Millennium Village. We hope they will prove a lasting
point of interest to visitors, and a lasting point of pride to the team who worked
so hard to produce the final, outstanding result.
A paper on the design and building of the three Tylers and Bricklayers will be
presented jointly to the 2001 British Sundial Society Conference by Piers Nicholson
and Lt. Col. Ian Ogden of the Corps of Royal Engineers. Two of the sundials form
part of the new Thames sundial trail in London, which is described on the Internet
at www.sundials.co.uk/~thames.htm
