what's the time?

Sundials

In ancient times, people used the Sun to tell the time. Noon or midday was when the Sun was highest in the sky and when it cast the shortest shadow. They were able to work forwards and backwards from there to mark the time according to the position of the shadow.

During the day the Sun moves in an arc across the sky. In the morning and evening, objects cast very long shadows. The time of day is judged by the length and position of the stick's shadow.In the northern hemisphere, the shadow is shortest when the Sun is directly in the south.

During the year the highest point in the sky reached by the Sun varies. The Sun reaches its highest point at noon on mid-summer's day and its lowest point at noon on mid-winter's day.

Unfortunately there are two further effects which make it difficult to use the Sun as an accurate time-keeper. The first is due to the fact that the Sun travels around the Sun at a variable speed (fastest in our winter and slowest in our summer). The 23° tilt of the Earth's axis also affects the way the Sun appears to move in the sky. Adding these two effects together means that the 'time' read from a sundial can be anywhere up to 16 minutes wrong compared to clock time.

To make a really accurate sundial that works all year round, the design has to allow these two effects to be corrected for.

Sunlight in the Arctic and Antarctic

At the Earth's south pole the Sun is above the horizon for about six months and below it for six months. Sundials are not much use there for most of the year! On the antarctic circle (latitude 66.5 degrees south), the Sun skims along the horizon all day on mid-winter's day, just touching the horizon even at midday. In between the antarctic circle and the south pole, the number of days with constant sunshine varies from one day to six months. The same of course happens in the arctic, but in reverse. Antarctic summer happens at the same time as arctic winter.

Photograph by Eric Woehler