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
