Deviation observed between the duration of the true solar days and the average solar day, period 1974 to 2005, measured in milliseconds, with respect to the theoretical value of 86 400 seconds.
The average solar day is an average of the true solar day, and corresponds to the civil time. Equivalent to 86 400 seconds, unit that is currently defined from very precise atomic properties, which allows to measure the differences with the true solar day.
This time is not measured directly by any type of clock but is obtained indirectly from the observation of other times: for example, the solar time ascertained by reading on the scale of a solar quadrant and calculated arithmetically by the equation of the time.
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