# What model best describes the alignment of the Sun and Moon during a lunar eclipse?

In the first image the penumbra shadow area is rather small and would match the time length of a lunar eclipse, However, the light source/Sun is far too distant to match the Sun in scale to the Earth in the present solar system model. The second image has the light source/Sun scaled more to the solar system model but the penumbra shadow is much to wide.

• Neither image is to scale, thus neither is realistic in this sense. Jan 4 '16 at 9:19

It's difficult but not impossible to model the Earth, Moon and Sun, but the scale is a bit tricky to get right if you try to build or draw it.

If we call the Moon's diameter = 1 moon unit, then, Earth's diameter is about 4 moon units. Distance between Earth and Moon, about 110 units, Sun's diameter, about 400 units, and the big one, the Sun's distance to the earth, about 43,000 moon diameter units.

So if the Moon is big enough to see, but still very small, say 1 mm, then the sun needs to be 40 cm in diameter (about 33% larger than a basketball) and 43 meters away.