diamond geezer

 Monday, February 23, 2004

About time [365]
The earliest form of clock was the sundial - a stick whose shadow marked the passing hours of the day. However, sundials are a surprisingly unreliable means of telling the time. Not just because it has to be sunny to use one, but because the Sun isn't always where you'd expect it to be. In fact a sundial only tells the correct time on four out the 365 days of the year.
Here's why. You might expect the Sun to be at its highest point in the sky at noon, but this isn't usually the case. The 23½º tilt of the Earth's axis places the Sun on the meridian at noon only at the solstices and equinoxes. Inbetween these dates the Sun runs either slightly fast (equinox to solstice) or slightly slow (solstice to equinox). But there's another factor at play here. The orbit of the Earth is an ellipse, not a circle, which places the Sun on the meridian at noon only at perihelion and aphelion. Inbetween these dates the Sun again runs either slightly fast (July to January) or slightly slow (January to July). You have to combine these two effects to get the true picture, producing a wobbly graph called the equation of time. The changing position of the noonday Sun therefore traces out a figure-8 pattern in the sky as the year progresses, a figure-8 called the analemma. There's a very good 1-page explanation of all this here, and a beautifully designed techie explanation here, complete with amazing moving graphics.
Sundials therefore run up to 14 minutes slow (in mid-February) or up to 16 minutes fast (in early November). And the only four days on which sundials tell the correct time? They'd be April 16th, June 14th, September 2nd and December 25th. Near enough. See all the figures for 2004 here.

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