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.
...or read more in my monthly archives
Jan26 Feb26
Jan25 Feb25 Mar25 Apr25 May25 Jun25 Jul25 Aug25 Sep25 Oct25 Nov25 Dec25
Jan24 Feb24 Mar24 Apr24 May24 Jun24 Jul24 Aug24 Sep24 Oct24 Nov24 Dec24
Jan23 Feb23 Mar23 Apr23 May23 Jun23 Jul23 Aug23 Sep23 Oct23 Nov23 Dec23
Jan22 Feb22 Mar22 Apr22 May22 Jun22 Jul22 Aug22 Sep22 Oct22 Nov22 Dec22
Jan21 Feb21 Mar21 Apr21 May21 Jun21 Jul21 Aug21 Sep21 Oct21 Nov21 Dec21
Jan20 Feb20 Mar20 Apr20 May20 Jun20 Jul20 Aug20 Sep20 Oct20 Nov20 Dec20
Jan19 Feb19 Mar19 Apr19 May19 Jun19 Jul19 Aug19 Sep19 Oct19 Nov19 Dec19
Jan18 Feb18 Mar18 Apr18 May18 Jun18 Jul18 Aug18 Sep18 Oct18 Nov18 Dec18
Jan17 Feb17 Mar17 Apr17 May17 Jun17 Jul17 Aug17 Sep17 Oct17 Nov17 Dec17
Jan16 Feb16 Mar16 Apr16 May16 Jun16 Jul16 Aug16 Sep16 Oct16 Nov16 Dec16
Jan15 Feb15 Mar15 Apr15 May15 Jun15 Jul15 Aug15 Sep15 Oct15 Nov15 Dec15
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
Jan04 Feb04 Mar04 Apr04 May04 Jun04 Jul04 Aug04 Sep04 Oct04 Nov04 Dec04
Jan03 Feb03 Mar03 Apr03 May03 Jun03 Jul03 Aug03 Sep03 Oct03 Nov03 Dec03
Jan02 Feb02 Mar02 Apr02 May02 Jun02 Jul02 Aug02 Sep02 Oct02 Nov02 Dec02
