The Cottesloe Sundial
Bicentenary Monument
Cottesloe is a small town on the West coast of Western Australia, just North of Freemantle where the Swan River meets the Indian Ocean. The bicentenary of Australia was celebrated in 1988. It marked 200 years since the arrival of the First Fleet of British convict ships at Sydney in 1788. Cottesloe Council decided to mark the bicentenary with a monument, and opened an architectural competition, which was won by Considyne & Griffiths Architects Pty Ltd with their proposal for a large sundial on the west-facing cliffs near the breakwater.
Having won, C&G P/L realised that they needed some expert input to make a working sundial. They recruited astrophysicist Tony Hooley who was at that time working for W.A.I.T. in Perth,
High Precision
Hooley only took on the job on the basis that it was an opportunity to do something new and better than anything else in the World. He designed a built-in correction system that freed the dial from the errors caused by The Equation Of Time in most other sun-dials (errors caused by the ellipticity of the Earth's orbit and by the tilt of its axis relative to the plane of the ecliptic).
The Equation Of Time
The sundial is based on the form of the ancient sundials in Jaipur, India, created by the Maharaja Jai Singh in the 18th century. There are two huge masonry triangular gnomons one of which casts a shadow on a massive cylindrical stainless-steel plate before noon, and the second similarly after noon. These plates are laser-engraved with a correction curve based on The Equation Of Time which completely removes these errors. The large size of the dial provides great precision, and the correction curves provide great accuracy. This sundial is the one of the largest in the Southern Hemisphere and the most accurate in the World throughout the year.
Construction
Groundwork
The surveyors were asked to layout the footings with extreme precision -- limestone masonry walls were then built with reasonable accuracy on the footings, but by providing precision-cast concrete capping sections which could be independently aligned atop the masonry walls, the overall precision could absorb errors in the walls. A series of concrete pillars was cast to support the stainless steel shadow-plates, which were attached with X/Y/Z adjustable fixings, allowing final calibration to minimise any remaining errors in the masonry work. So that the dial would read Australian Standard Time (rather than a somewhat inconvenient Local Solar Time), the calibration marks on the dial correct for the location of the sundial which is 4 degree 14min 59sec West of the 120deg E meridian.
The Shadow Plates
This view shows the mountings of the shadow plates on the concrete pillars
SIte Location
This view from Cottesloe breakwater clearly shows the water's-edge position of the sundial atop a low limestone cliff
View from the South
The paving around the sundial has been laid in a wavy pattern emulating The Equation Of Time.
Me And My Shadow-Plates
Equation Of Time
Hooley in Oz-mode beneath the western shadow plate, on which can clearly be seen the Equation-Of-Time engraved correction curves, which are the sum of two sine waves with periods 12 months and 6 month
Eastern Shadow Plate
The location of the sundial on the cliffs facing West into the Indian Ocean means this sundial can track the Sun's shadow until it sets on the horizon. The gap in this end of the plate is to allow the rising Sun's rays to pass through from behind to fall onto the Western shadow plate at the Equinox.
Eastern Shadow Plate
As can be seen in the first and third LARGE photos above the end of the Western shadow plate has a complementary taper so that, at the Equinox, the Sun's rays can pass either side to strike the upper tips of the Eastern shadow plate at sunset.