Hello Peter,
I have a graph (URL below) illustrating the effect of atmospheric
refraction as a function of the Sun's geometric ("true") altitude. It
shows that the effect is realistically never more than 0.5deg and after
approx. 10Degs of geometric altitude the effect is negligible.
ftp://ftp.gcstudio.com/pub/sundial/refrac.gif
Regarding the graph of the analemma as compared to the graph of the
equation of time (EoT), the analemma is a graph of the EoT (x-axis) vs
solar declination (y-axis), the graph of the EoT is typically the EoT
(y-axis) vs the date (x-axis). Also, the EoT is typically expressed in
the form (apparent_time - mean_time) (TA-TM) BUT not always (esp. in
older books). Here again I'll appeal to an image, the URL(s) below will
illustrate the relationships I've just described, note however that both
the EoT and analemma images are expressed in the form TM-TA, (yeah, I
know!).
ftp://ftp.gcstudio.com/pub/sundial/sday_eot.gif
ftp://ftp.gcstudio.com/pub/sundial/analemma.gif
Best,
Luke Coletti
Peter Hirtle wrote:
>
> Has anyone studied the effects of atmospheric refraction on the accuracy
> of a sundial? I'm thinking in terms of a heliochronometer where errors
> of less than a half minute are significant.
>
> Another thing I would like to know is if there is an analogy that makes
> the equation of time and the shape of the analemma "intuitively" obvious?
>
> Peter
>
> --
> Peter Hirtle [EMAIL PROTECTED] Seattle, WA.
