Hello Willy,
The model I find useful actually applies to both effects, i.e.,
both eccentricity (the elliptical shape of our orbit) and obliquity (the
tilt of our axis relative to the plane of our orbit). Namely, the Sun's
daily position relative to the background of fixed stars appears to move
in a eastward direction and at a variable rate. If the Sun's apparent
motion eastward is variable then so to is the length of our apparent
solar day. The daily variation in the length of our solar day is
additive, the sum of which is the Equation of Time.
The question then becomes how do each of the two effects
(eccentricity and obliquity) produce this change in the apparent
eastward motion of the Sun. Obliquity causes this apparent motion to
vary because the Sun in its apparent eastward migration will move along
a path that is not always parallel to the direction of the Earth's
rotation. At the Solstices the track of the Sun IS parallel to the
direction of the Earth's rotation and the Sun appears to move fastest in
its apparent eastward direction resulting in the apparent solar day
being longer. The opposite is true at the Equinoxes.
There are several ways of describing the relative relationship(s) but I
like this one best.
-Luke
Willy Leenders wrote:
>
> The equation of time has two causes. The first is that the orbit of the earth
> around the sun is an
> ellipse and not a circle. The second is that the plane of the earth's equator
> is inclined tot the
> plane of the earth's orbit.
> Please can anyone explain me the second cause so that I can conceive it. I am
> not a astronomer!
>
> You can do it in Dutch (for preference), in French, in German or in English.
>
> Willy Leenders
> Hasselt
> Belgium
