Luke Your statement that eccentricity will always be synchonous to the passage of perihelion needs, I believe, amplification. There is no a priori reason that I know of that the mean sun and the actual sun should have the same right ascension at perihelion. The fact is that one may choose the mean sun to have the same right ascension as the sun at any time of the year. The resulting EoT of course will be different and will not average out to zero during the year. There is a point in the year such that, if the mean sun corresponds to the right ascension of the sun, the EoT will average out to zero. It just happens that this occurs very near to perihelion. My calculations show that the position is within a fraction of a degree of perihelion but is not at perihelion. There may be an argument that it should be at perihelion but I do not know that argument. The calculation that I did to find that point where the average EoT goes to zero may be round off error dependant.
Can you find my error in thinking or can we agree.? Dan Wenger >Hi Bill, > > If you mean to ask why the EoT was made to be zero at a given >set of dates, I think the answer is that it wasn't. One can't >arbitrarily make the EoT zero points (four of) synchronous to a set of >dates. The EoT has two components, obliquity (the tilt of our axis >relative to the plane of our orbit) and eccentricity (the elliptical >shape of our orbit). Obliquity will always be synchronous to the four >cardinal positions of the orbit (the equinoxes and solstices), >eccentricity will always be synchronous to the passage of perihelion. >The two components however are NOT synchronous to one another, I have >explained this in some detail in earlier messages. In short, because the >two components are not synchronous to one another the EoT undergoes >variation in time. So a set of 17th century values will definitely not >be the same as those today, i.e., the shape of the analemma will be >different. > >Regards, > >Luke Coletti > > >[EMAIL PROTECTED] wrote: >> >> 4/3/00 >> Does anyone know why the equation of time is indexed to zero on 9/1, 12/25, >> etc.? That is to say, when the clock was originally being indexed to >>the sun >> (17th century?), why did they pick this set of dates as the zero point? Why >> not, for example, set the clock to where the analemma crosses itself, or to >> one of the solstices, or equinoxes? I'm sure there is a good reason, but I >> haven't been able to think of it. Maybe it has to do with indexing the >>clock >> to sidereal time, and not to sun time. Any takers? >> >> Bill Gottesman >> Burlington, VT Daniel Lee Wenger Santa Cruz, CA [EMAIL PROTECTED] http://wengersundial.com http://wengersundial.com/wengerfamily
