> When I opened
> the file and saw the diagram with
>  (Sun)----(Moon)----(Sun0)----(Moon0)----(Earth)
> I realised I am going to have to disect this file. Then when I saw you have
> the sun and moon rotating around the earth, and knowing this is only to find
> eclipses, I figured I would have to do some "reverse engineering" as well.
>
> This could work but I need to know which  Sun and Moon are correct?
>
> Thanks.
>
>
> --
> -Nekar Xenos-

I started by trying to create a realistic scene with the Sun, Moon and
Earth.

The Sun was originally an extended light source.

This gave me the effect seen in the simple animation on the home page
http://www.pavium.info/Eclipse

The shadow was fuzzy (as it should be) but this made it difficult to see the
path of totality.

I wanted to see if I could reproduce the maps included in NASA's literature,
so I came up with an artificial solution.

I made the Sun a point source of light. This gave the Moon's shadow a sharp
edge. Then I added a second sun (Sun0) between the Moon and Earth and a
second TINY moon (Moon0) between Sun0 and the Earth. Sun0 dimmed the shadow
cast by the Moon, but Moon0 added a small shadow which represented totality
in the centre of the shadow made by the true Sun.

Here's a diagram. (Light travels left to right)

   True     True
  (Sun)----(Moon)----(Sun0)----(Moon0)----(Earth)

    |                   |                   No Shadow
    |        |          |                   Shadow of Moon  = penumbra
    |        |          |                   Shadow of Moon  = penumbra
    |        |          |         |         Shadow of Moon0 = umbra
    |        |          |                   Shadow of Moon  = penumbra
    |        |          |                   Shadow of Moon  = penumbra
    |                   |                   No Shadow

If you just want simple, natural, shadows you should get rid of Sun0 and
Moon0, and change the definition of Sun to give you a light emitting sphere
of radius 6.96E6 kilometres.

Along the way, I decided that a point-source Sun required a double-size Moon
to give a 'penumbra' of the right size. This was really a trial and error
thing, I have no proof, it looked right - but you should look for this and
make sure the Moon is the correct size.

Actually, I'd forgotten about the 'geocentric co-ordinates' and the
impression it gives that everything rotates around the Earth.

It's just convenient mathematically to move the origin of the co-ordinates
to the Earth (especiallly since that's where the camera is pointing). I get
the impression astronomers switch co-ordinates systems at the drop of a hat.

'Astronomical Algorithms' provide routines to calculate the orbits of the
planets in geocentric co-ordinates, and it can be a lot simpler than the
calculation of the Moon's position.

Regards,
pavium

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