“wHILE PER­PET­uAL CAL­EN­DARS ARE AL­READy IM­MENSELy COM­PLEx, THE PIN­NA­CLE Of CAL­EN­DAR COM­PLI­CA­TIONS IS, wITHOuT DOuBT, THE EquA­TION Of TIME”

DA MAN - Caliber - - MECHANISM -

words, na­ture’s time) and “mean” so­lar time (a man­made con­struct—what we, hu­mans, count as a full day). Please bear with us as we at­tempt to ex­plain.

See, the Earth’s or­bit around the sun is el­lip­ti­cal. Not only that, the axis of the planet’s ro­ta­tion is slightly tilted from per­pen­dic­u­lar to the plane of the equa­tor. As a re­sult of th­ese two fac­tors, the length of a “true” so­lar day (which is de­fined as the in­ter­val be­tween two “true” noons—when the sun is at its high­est point in the sky) varies quite a bit through­out the year. In fact, there are only four days in the year that are ex­actly 24 hours: April 15, June 14, Septem­ber 1 and De­cem­ber 24. All the other days are ei­ther shorter or longer than 24 hours, rang­ing from mi­nus 16 min­utes and 23 sec­onds (on Novem­ber 4) to plus 14 min­utes and 22 sec­onds (Fe­bru­ary 11).

His­tor­i­cally, sun­di­als would show “true” so­lar time, and some early clocks were equipped with mech­a­nisms that would cor­rect their read­ings to match sun­di­als. As me­chan­i­cal clocks be­came our stan­dard time­keep­ing de­vice, “mean” time (which was tech­ni­cally “un­cor­rected”) be­came the ac­cepted stan­dard. Equa­tion of time watches (with the word “equa­tion” here is used in the ar­chaic sense of “rec­on­cil­ing a dif­fer­ence”) once again al­lows us to fol­low “true” time.

Most equa­tion of time watches show the dif­fer­ence be­tween “true” time and “mean” time on a sep­a­rate sub-dial (usu­ally an arc)

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