Sundials

Sundials and the Equation of Time

Sundials are literally as old as time. Age-old mysteries and wonders inherent in the movement of sun-shadows echo in them.

No one knows when humans first learned to tell time, but the art of the sundial was developed by the ancient Egyptians, Babylonians and Greeks. They created this time-telling tool by observing the apparent movement of the Sun across the sky and its position at a certain moment. The word 'apparent' is crucial in understanding how time measurements started their evolution towards today's atomic clocks.

Apparent solar time and mean solar time

Looking at the sky might give someone from Earth the impression that the Sun is moving from east to west. In fact, the Earth is moving around the Sun, taking a whole year to complete its journey. To make things even more complex, it's not a circular journey or orbit, but an elliptical one.

Moreover, the Earth rotates on its own axis. That rotation takes 24 hours, but its axis is tilted, so at different points during the year what is visible in the sky will change.

Apparent solar time is a daily clock that resets itself once the Sun rises, while mean solar time is the average of these daily occurrences over a whole year.

To each daylight hour, its own goddess

Ancient Rome measured time by dividing the day in two parts, the sunlit one and its dark counterpart. They were divided into 12 intervals, with varying length. In the summer time, more daylight meant longer hours, as there was more time to divide by 12. Moreover, each had a name, a precise function and a deity.

What would modern cities go by if this system were applied today? We give you the Horae Timepiece!

The Equation of Time

The difference between apparent solar time, as shown by sundials, and mean solar time, as expressed by Greenwich Mean Time (GMT), is called The Equation of Time.

Its value is zero only four times a year, mid-April, in June, September and December.

Greenwich Mean Time, based on average length of the solar day as measured at Greenwich, England, on the Prime Meridian, can differ by more than 10 minutes from time shown by a sundial. Sundials can be as much as 14 minutes behind or 16 minutes ahead of GMT. That led to the creation of conversion tables between the two values.

Sundial Trips

A most interesting Thames Sundial Trail goes from the Houses of Parliament (Big Ben) down the River Thames through Greenwich to the Thames Barrier. This route takes in some of the best views of London, the capital of the UK.