Hi Diallists, I think Gianni's excellent postings show the cause of our dilemma on the EoT sign convention.
As traditional diallists, should we take the LAT (solar) time as the basic value, and add EoT to it to give the artificial Mean Time? Or, as good scientists, should we adopt the astronomers' convention and take universal Mean Time as the basic value and add EoT to it to derive solar time? The BSS Glossary prefers the former, although it does note the latter. After all, many early eighteenth century dials, which probably represent the height of the dial making art, have an EoT ring which shows the values as "Watch faster" and "Watch Slower" i.e. the correction is to be added or subtracted to solar time. I don't think we will ever convince everyone to adopt a single convention. Even if we did, we are stuck with a wide historical literature which is fairly evenly spilt between the two possibilities. So we must always read with our brains engaged, and make new diallists aware that they are likely to encounter both options. Happy dialling, John Davis > 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 superseded in civil use by mean solar time.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 > > >
