16 April 2018 by Roy Preece
The earth doesn’t go around the sun in a perfect circle, so the length of the day – noon to noon – in fact changes throughout the year. For convenience our clocks show the average length of the day as Greenwich MEAN time (get it?). Sunday, April the 5th was one of the rare days in the year when the sun and the clocks agree. Some people suggested it was a good time to adjust your sundial to show the same time as your watch!
That’s strange! In the days before accurate digital timepieces, people would use the sundial to correct their clocks! And they didn’t need to wait for a perfect day as The Astronomer Royal produced tables to show the time difference for every day of the year.
I wonder, did the legendary Wan Nian know this too (Wan Nan’s Calendar)? The Chinese built giant very accurate sundials, but legend says Wan Nian gave up sun dials when his wife pointed out the sun doesn’t always shine, and chose water clocks instead.
It is said the ancient Greeks had water clocks which so accurate that they could be adjusted for the changing length of day. Amazing!