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Sundial

You will find the Sundial of Human Involvement just off Caley Drive — stop at the sign and walk up the path on the right hand side of the road to Sundial Hill, located near a saddle of Mount Annan. From here there is a spectacular 360 degree view of the surrounding countryside. On a clear day you will see the city of Sydney to the north-east and the Blue Mountains to the west

A rare type of sundial

The Sundial of Human Involvement is a rare type of sundial that uses a person’s body to cast a shadow onto hour markers and to tell the time. The position of the person is adjusted throughout the year following  a figure of eight pathway, or analemma — so this type of sundial is known as an analemmatic sundial.

Telling the time

Follow the instructions on the nearby bronze plate. When your feet are on the correct date, clock time is given by the position of your shadow, providing you are tall enough — you might have to raise your hands above your head to become a bit taller! Check the time on the conventional horizontal sundial (accurate to within 1 or 2 minutes) on your right, but remember to follow all the instructions correctly!

sundial instructions

How accurate is the Sundial of Human Involvement?

The simple method of telling the time using the Sundial of Human Involvement does not give precise clock time throughout the whole year. However, the small errors lie within the width of the shadow and occur in the early morning and late afternoon at times of the year corresponding to the maximum and minimum of the equation of time (see below). The novelty value of being able to read clock time directly from a sundial well compensates for the slight mathematical imperfections introduced!

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Sundial Hill

 

The hour markers
The basalt crystals used as hour markers in the Sundial of Human Involvement came from the Blue Mountains Botanic Garden. These crystals are featured in the construction of the walls within the Garden. The tops of the basalt columns used for the sundial were cut off at an angle of 35 degrees, and the hour numbers were chiselled into the stone. The polished faces have been waxed to improve their appearance and make the numbers more clearly visible.

The analemma plate
The basalt slab for the analemma came from Port Fairy in Victoria. This basalt originates from the volcanic region of south-west Victoria and is a dark grey-black colour, with little crystal face reflection when broken. In colour, texture and gas bubble content it is remarkably similar to the basalt columns from Mount Tomah.

Solar time and clock time
Most other types of sundial show ‘solar time’ and you need to make certain corrections before the sundial will show the same time as indicated on your watch. The first correction, called the equation of  time occurs because of the tilt of the earth’s axis of rotation and because of the elliptical shape of the earth’s orbit around the sun. The sum of these effects varies with the date but is the same everywhere on earth. The second correction occurs because of the difference in time between the sundial's location and the local time zone longitude. This correction is constant throughout the year, but is different for different locations.

The Sundial of Human Involvement incorporates the effects of longitude into the position of the basalt crystals used as hour markers, while the equation of time corrections are built into the central analemma.

Design of the Mount Annan sundial
The Sundial of Human Involvement was designed by Dr Margaret Folkard and Mr John Ward of Sundials Australia in Adelaide. The stone was carved by Adelaide sculptor Mr Silvio Apponyi. You can see two other different types of sundials, also designed by Sundials Australia, at our other Gardens — an equatorial sundial at the Blue Mountians Botanic Garden and an armillary sphere sundial at the Royal Botanic Garden.

Other analemmatic sundials
Other analemmatic sundials may by found at the: 
Cathedral of Brou near Bourg-en-Bresse, 370 km south-east of Paris, France;
Great Exhibition Hall in Liverpool, England;
Reinauen Central Park near Bonn, West Germany; 
Park near the Palace of the Popes, Avignon, France;
University of Arizona, Tucson, USA; and
University of North Carolina, Charlotte, USA.

Making your own analemma
Find a room that has a wall with a window facing approximately north. Cover an area of window about 400 mm square with black cardboard and cut a neat hole in the centre of the cardboard about 12-15 mm in diameter. The hole should be about 1.5 to 3.0 metres above the floor. Mark the position of the sun’s image on the floor (or wall) at the same time of the day, say 12 o’clock on your watch, every few days for a whole year. After a year you will have an analemma traced out on the floor. By repeating this exercise at different times, say 9 o’clock, 10 o’clock, etc., you will get a family of analemmas.

Instead of a hole you can use a mirror about 50 mm square (or round) located on a window sill to produce analemmas on the ceiling and walls of a room. 

Layout of the sundial
The Sundial of Human Involvement records time with respect to the azimuth of the sun. The azimuth is the angular distance on the horizon plane between the true north-south line and the foot of the perpendicular from the sun to the horizon.

The Sundial of Human Involvement consists of an elliptical ring of basalt crystals, which represent the hour markers, with a major axis of about 5 meters and a minor axis of 3 meters. Each hour marker has a number carved in it. Inside the ellipse, and located along the north-south line, is an analemma with dates marked on it at fortnightly intervals around the circumference.

A gift to the Garden
The Sundial of Human Involvement is dedicated to the memory of Winifred Macarthur-Onslow, who lived in and loved this countryside, and is a gift to the Australian Botanic Garden from her daughters. The Macarthur homestead can be seen in a south-west direction from the sundial.