During a given day the Sun appears to move across the sky from East to West, causing continual changes in both the length and position of the shadow cast by any object. The seasonal change in the Sun’s height above the horizon causes further changes in the shadow’s position.
Sundials in one form or another have been used by different societies for more than 5000 years. From ancient beginnings, methods of designing and constructing various types of sundials have been developed by different cultures right through to the present day. The Greeks and Romans in particular created a great variety of sundials and used them throughout their vast empires. As an example, in 150AD, Ptolemy of Alexandria stated in his theory that the Earth was stationary and at the centre of the Universe, and went on to draw hour lines of a sundial by projection, and invented the analemma.
The sundial you see at GZ is called an analemmatic sundial. The analemma is the figure 8 shape you see associated with these types of sundials – in this case, you can see a stretched 8 shape cut into the metal plate on the ground at GZ. Typically these use a movable vertical post to cast the shadow onto a horizontal scale. The hours can be seen to be marked on series of limestone rocks containing local fossil artwork around the circumference of an ellipse to the southern side of the ground plate. This is quite different from the more common sundial that shows the hour lines radiating from the toe of the fixed gnomon (the bit that casts the shadow).
Because the Sun’s position varies with the seasons, the vertical “pin” (the gnomon) must be set in a different position along a north-south axis for each day of the year. In the case of this lovely sundial, you can use your own body to be the gnomon, so really this type of sundial could be known as a “human sundial”. Accuracy is limited by the width of your hand and where you stand on the analemma.
The biggest problem with sundials purchased over the counter eg in garden shops is that they are never, except purely by accident, designed for the location at which they are to be used. This has led to the idea that sundials do not tell the time correctly. Nothing could be further from the truth. When a sundial is designed for an exact latitude and longitude, sundials can readily give time readings accurate to one minute or better.
Assuming that there are no major geological processes occurring over short time frames (less than 10,000 years), well constructed sundials can tell accurate local time for thousands of years. Due to the Earth’s precession (26,000 year wobble like a spinning top) and plate tectonic movements, even well constructed and robust sundials will eventually not tell the correct time. This sundial is designed for Parachilna, and is very accurate, and you can test this.
PLEASE SEND YOUR ANSWERS TO THE FOLLOWING QUESTIONS:
- Can you carry a sundial with you to use it to tell the time from anywhere on the Earth? (assume the Sun is shining!)
- Stand on the analemma dot that corresponds to the date you are here. Raise your hand and comment on the accuracy of the time given by the sundial compared to the real clock time. Does daylight saving need to be accounted for here?
- What compass direction does your shadow fall in question 2? Would this be different if the sundial was located at latitude 31°08’ North? If so why?
- Take a photo of your feet as you stand on the dot (as in question 2). Please upload this photo with your log. Logs without this photo will be assumed to be armchair logs and will be deleted!