Monday, February 23, 2004
About time 
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.
...or read more in my monthly archives
Jan15 Feb15 Mar15
Jan14 Feb14 Mar14 Apr14 May14 Jun14 Jul14 Aug14 Sep14 Oct14 Nov14 Dec14
Jan13 Feb13 Mar13 Apr13 May13 Jun13 Jul13 Aug13 Sep13 Oct13 Nov13 Dec13
Jan12 Feb12 Mar12 Apr12 May12 Jun12 Jul12 Aug12 Sep12 Oct12 Nov12 Dec12
Jan11 Feb11 Mar11 Apr11 May11 Jun11 Jul11 Aug11 Sep11 Oct11 Nov11 Dec11
Jan10 Feb10 Mar10 Apr10 May10 Jun10 Jul10 Aug10 Sep10 Oct10 Nov10 Dec10
Jan09 Feb09 Mar09 Apr09 May09 Jun09 Jul09 Aug09 Sep09 Oct09 Nov09 Dec09
Jan08 Feb08 Mar08 Apr08 May08 Jun08 Jul08 Aug08 Sep08 Oct08 Nov08 Dec08
Jan07 Feb07 Mar07 Apr07 May07 Jun07 Jul07 Aug07 Sep07 Oct07 Nov07 Dec07
Jan06 Feb06 Mar06 Apr06 May06 Jun06 Jul06 Aug06 Sep06 Oct06 Nov06 Dec06
Jan05 Feb05 Mar05 Apr05 May05 Jun05 Jul05 Aug05 Sep05 Oct05 Nov05 Dec05
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