Hi Piers , I have visited with a lot of interest your Solar Noon Calculator on the web at www.solar-noon.com and I have immediately made some tests to compare your values with those calculated from me and published in an article in the proceedings of our "X Seminario di Gnomonica" ( X Italian Meeting on Sundials - 2000) Here are some considerations of mine
1) >From the comparison I have immediately seen that your results are wrong because of a banal error: anywhere the value of the EoT has been or taken with opposite sign or subtracted instead that added. An example : Long. =12d E and TZ of Central Europe (central Meridian 15 d East): longitude correction =+3d = +12m At 1/1/2001 the exact value of the Eot = - 3m41s an so : - Local Apparent Time (apparent solar time) = 12h - Local mean time (local mean solar time) = 12h 3m 41s - Standard Time = 12 15m 41s (NASS Dialist Companion gives the value, less approximate, 12h 15m39s ) In the Table calculated with the Solar Noon Calculator is written the value = 12h08m48s, value that is obtained adding (instead that subtracting ) the value of the Eot: 12h+12m+(-3m12s) = 12h08m48s 2) The table of the EoT NOT gives the values of the EoT but the Total correction that it is necessary to add to the Local Apparent Time to obtain the Standard Time : it is therefore the sum of the EoT + longitude correction.. This value is certainly very useful but, perhaps, it is necessary to give some explanations and it is opportune not to call it Eot Moreover in this way the table with the values of noon is useless because these values are equal to those of the EoT + 12h The definition : "Equation of Time displays the difference between solar time and the standard times where you are" (note at the foot of the page) it is not correct. Davis' Sundials Glossary gives the following: Equation of Time: the time difference between Local Apparent Time (apparent solar time) and mean solar time at the same location (NOT Standard Time). Its value varies between extremes of about +14 minutes in February and -16 minutes in October. 3) Checking only for the date 1/1/2001 I have found that the error between the exact value and the mean value of the EoT = 3m41s -3m12s=29 sec: almost the double of the maximum error (in the Note). With NASS Dialist Companion we obtain the less approximate value Eot = 3m37s: also with this value the error would be of 25s Does the greater error depend on the fact that the Eot has been calculated at 0h (UT) instead that at 12h (UT) In fact the Eot can also change till 20s a day. 4) In my opinion it is very useful, for instance in the construction of sundials with mean time, to have a table of the mean values of the Eot (as of the mean declination of the Sun), while the table that gives the mean local noon (mean on 4 years) can be used in a wrong way. In fact these tables, that should be used only for the search of the mean time from the apparent solar time given by a sundial, could be considered right also in the search of the declination of the walls. In this case it is better to use the true value of the EqT calculated with programs as NASS Diallist Companion Perhaps a note could clarify the thing. 5) As in almost all Web sites, also you take as positive the Longitudes for places West of Greenwich. Despite the opinion of the known astronomer J. Meeus, with which also Davis agrees in his Sundial Glossary, even if a secular tradition justifies this definition, it is NOT correct. The Explanatory Supplement to the Astronomic Almanac (USNO 1992) at page 203 affirms: "The geocentric longitude is defined by the angle between the reference (or zero) meridian and the meridian of point p, measured EASTWARD around the Earth from 0 to 360 deg (IAU, 1983, p.47) " Perhaps it would be opportune, at least in new programs and in new Web sites , to use this correct definition Best wishes Gianni Ferrari
