equation of time
From Glossary of Meteorology
The difference at any instant between the apparent solar time and the mean solar time as measured at a specified place; it is the difference between the hour angles of the apparent sun and the mean sun.
Tables of the equation of time are available in astronomical almanacs. The equation of time is zero at four times during a year, on about 15 April, 15 June, 31 August, and 24 December, at present. The algebraic sign associated with the equation of time varies from one source to another, a fact that must be kept in mind in abstracting values from almanacs.
See apparent solar day, mean solar day.
