By mistake, I have written that in the Explanatory Supplement to the Astronomical Almanac (1992) the Equation of Time is defined as ( Mean Time - Local Apparent Time ) This is not true! As in all other books of astronomy also in ESAA (pages 5, 74, 727) we find EoT = AT - MT I apologize for this error
I transcribe below some lines from ESAA on the history of the EoT < Until the early nineteenth century the determinations of local apparent time were commonly made by observing altitudes of the Sun or stars. Thus apparent solar time was the argument in The Nautical Almanac and other national ephemerides. Mean time, when needed, was obtained by applying the equation of time to the apparent time. The equation of time, in the sense of the correction to be applied to apparent time in order to obtain mean time ( EoT=MT-AT) , was tabulated .... It was used for regulating clocks and determining the .argument for entering astronomical tables. (1) During the late eighteenth and early nineteenth centuries, as clocks were improved and came into extensive use at sea, apparent time was gradually superseded in civil use by mean solar time. In the mid-nineteenth century, when mean time was first introduced as the argument in the national ephemerides,..(snip)....the equation of time came to signify the opposite of the original concept. Apparent solar time was obtained by applying the equation of time to the mean time kept by clocks (EoT=AT-MT) , which were regulated by determinations of mean time from observations of sidereal time > Note (1) I think that we, keen on sundials, behave our self still as our great-grandfathers and we desire to find the Mean Time, and to adjust ours clocks, from the Apparent Time that our solar clocks mark. My conviction that is more opportune to use the formula EoT = MT - AT, is strengthened by the observation that in ALL the the sundials with the curve of the EoT, this has a maximum in January-February and a minimum (negative) in November. With a smile Gianni Ferrari
