Hello Jos,
As you know the equation of time (EOT), which you measure, is a unique
function of three parameters:
1) The tilt of the earth's axis towards the ecliptic
2) The distance (or time) between the perihelion passage of the
earth (around Jan. 4th) and the spring equinox (around March 21st)
3) The earth's eccentricity
Assuming that you measure the EOT and you know 1) and 2) you can find
the eccentricity by finding the best fit to your data. Theoretically you
need only 2 points on the EOT, but in practice you need of course a
large part of the EOT to get a good fit. Don't forget that you have to
correct you sundial time for the longitude of you dial before comparing
it to you watch.
In case the earth would orbit in a circle, the EoT would only be due to
the tilt of the earth's axis and the analemma would be a symmetric "8"
curve. The middle point of the "8" would be at the equinoxes. For me,
the 'symbolic', effect of the earth's eccentricity is therefore the
displacement of the middle point of the analemma towards positive
declination.
Regards Werner
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On Behalf Of Jos Kint
Sent: Thursday, August 21, 2008 10:12 AM
To: sundial@uni-koeln.de
Subject: Earth eccentricity
Hello all,
I am looking for some help in observing the earth's orbital
eccentricity, just by using my sun dial. Who gives me some hint? With my
vertical 2,5 meter by 1,5 meter sun dial I can measure the local solar
time with an accuracy of less than 60 seconds.
Jos Kint, Belgium
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