Equation of time
What is the equation of time?
The difference between clock noon and sundial noon. The Earth’s orbit is a slight ellipse and its axis is tilted, so the true solar day wanders by up to about 16 minutes through the year. Clocks run a mean 24 hours and ignore it.
Why a sundial is 'wrong'
In November a sundial can run a quarter-hour fast. In February it can run slow. The clock is the one that is smoothed. The sundial is following the real Sun.
Two effects add up: the orbit is not a circle, so Earth speeds up and slows down, and the axis is tilted, so the Sun’s path along the equator is not even. Together they are this equation.
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The analemma
Photograph the Sun at the same clock time every few days for a year and the spots make a figure-8. That shape is the equation of time plotted against the Sun’s changing height. It is the tilt and the ellipse, drawn in the sky.