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The Equation of Time

Originally posted on Dark Sky Diary by Steve Owens (@darkskyman on twitter)

Today, 13 June, is one of only four days in the year when the time as read on a sundial will be exactly correct.

Sundials usually tell the time using the shadow of the gnomon as cast by the Sun. This is possible as the Sun appears to move across the sky at an approximately constant speed, and so the shadow of the gnomon also moves at an approximately constant speed. The inconstancy of the Sun’s apparent motion in the sky – and therefore of the gnomon’s shadow on a sundial – is the subject of this article, and is calculated using the Equation of Time.

If you look at the shadow of a sundial’s gnomon it will fall onto a curve of numbers, along hour lines indicating local solar time. This is not equal to the official clock time until three important corrections are made:

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