In Italian Nautical Astronomy we were used to distinguish between Equation of Mean Time and Equation of Apparent Time (in Italian we call Apparent Time as True Time because it is referred to the true Sun shining in the sky!) . They are just the same numerical entity but the algebric sign is reversed so that EqMT = - EqAT and viceversa. In such a manner following formulas always carry the (algebric) sign plus
True Time = Mean Time + EqMT Mean Time = True Time + EaAT and were easy to be remembered by seamen. The sign to be given Equation of Time is by agreement and the rule has been changed along this past years and it seems to me that no ultimate agreement has been reached. Prof. Francesco Zagar in his book Spherical and Theoretical Astronomy (1948) says: "Equation of Time is the difference between Right Ascension of Mean Sun and Apparent (True) Sun..... Because differences in Right Ascensions has the opposite sign of respective differences in Hour Angles, EOT becomes EOT = AppTime-MeanTime (and so AppTime=MeanTime+EOT) ...so that EOT is the amount to be algebrically added to Mean Time to get Apparent Time" (original text is in Italian and translation is by myself). In a footnote on the same page he says that a few years ago (in respect with 1948) EOT was intended on the contrary. Same opinion from F.Flora (Nautical Astronomy - 1964) and G.Severino (Nautical Astronomy - 1971) which give the following values: 12 Febr EOT =-14m30s 14 May EOT = + 3m53s A.Nicoli (Astronomical Navigation - 1994) likes better to use Equation of Apparent Time but giving just the same values and sign as above. Jean Meeus in Astronomical formulae for calculator gives the following definition: "EOT is the difference between Right Ascension of Apparent Sun and Mean Sun" Note that is the contrary of F.Zagar, but for the example in the page he gives the negative value of -11m10sec for 21 January. Italian Nautical Ephemeris (by Italian Hydrografic Office) has changed, a few years ago, the sign of EOT so now for Feb 14th is given -14 m 20 sec. Explanatory Supplement to the astronomical Almanac, gives at page 727 the following: "equation of time: the hour angle of the true Sun minus the hour angle of the fictious mean sun; alternatively, apparent solar time minus mean solar time". So we have: AppTime=MeanTime+EOT At page 485 there is the graph of the annual curve and for February negative values are given. At page 75 a different definition is given: "...EOT is the correction to be applied to apparent time in order to obtain mean time..." from wich derives Mean Time=App.Time + EOT and AppTime=MeanTime - EOT but at page 55 there is a graph where we can clearly see that True Sun= Mean Sun + EOT As you can see there are different definitions about the same entity. Will you excuse me for my poor English Best regards ----------------------------**--------------------------- Franco Martinelli Viareggio (LU) [EMAIL PROTECTED] http://www.dex-net.com/EI886 ----------------------------**---------------------------
