Bart Bear's Blog

This thing that looks a like sail on a post in a concrete circle is actually a special sort of clock called a sundial. We found it in the park by the wharf where cruise boats are in Perf. The sail is what Dad calls a Gnomon (astronomers have a lot of these funny words). The shadow of the gnomon moves around the concrete circle as the Sun moves across the sky and the circle has marks on it that tell what time it is when the shadow is on the mark. Sundials are one of the oldest type of clocks and they are not completely accurate. This is because the Sun is higher in the sky in summer than it is in winter and is only due north at midday four times a year (due to things about the Earth's orbit that the Oldies understand but small bears don't yet). That means the shadow can be running fast or late when it reaches the mark. You can tell the correct time if your sundial has a diagram on it that shows how fast or slow your sundial is during the year. On this one it is a graph on a bronze plaque near the mast. Scruffy and I found it and figured out the time and it even agreed with what Dad's watch said. Sundials are clever things but watches are easier to wear and they work even when the Sun is behind clouds and at night.

Labels: astronomy, Perth, Western Australia