Equation of Time
The difference between mean and apparent time —i.e. the difference between the time as shown by a good clock and that indicated by a sundial. The greatest difference is in November, at the beginning of which month the sun is somewhat more than sixteen minutes too slow. There are days in December, April, June, and September when the sun and the clocks agree.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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