figure on page 61, showing phases of the moon

[BVD]

Mom: I loved your book, and now my 9-year old son is reading it, and enjoying all your puns and bad jokes (not to mention the science!).

Son: The figure on page 61 seems to imply that every time the sun, moon and/or earth line up, there should be either a solar or lunar eclipse. Can this figure be redrawn to show what actually happens when they line up? Why doesn't an eclipse happen every time they're in line?

[SeanF]

On 2002-07-11 11:16, BVD wrote:
Mom: I loved your book, and now my 9-year old son is reading it, and enjoying all your puns and bad jokes (not to mention the science!).

Son: The figure on page 61 seems to imply that every time the sun, moon and/or earth line up, there should be either a solar or lunar eclipse. Can this figure be redrawn to show what actually happens when they line up? Why doesn't an eclipse happen every time they're in line?

I don't have my copy of the book with me, but keep in mind that the actual orbit is three-dimensional, and it's tilted relative to the ecliptic - the plane of earth's orbit around the sun. If we figure that the paper represents the ecliptic and the earth and sun both sit "in" the paper, then the moon will spend about half it's time above the paper and half it's time below the paper.

An eclipse only occurs when the moon crosses the plane of the earth's orbit at the same time as it crosses the line between the earth and the sun.

I can't think of a real easy way to draw that with a single diagram, but does it make sense the way I've described it?

[BVD]

Thanks for the description. The discussion has spawned some debate here, which is a good thing, and some interesting experiments to try. How about changing the entry angle of the sun's light in the diagram, so that the two-dimensional representation does not look like the earth and moon are constantly in same elliptic as the sun?

[GrapesOfWrath]

On 2002-07-11 11:16, BVD wrote:
Son: The figure on page 61 seems to imply that every time the sun, moon and/or earth line up, there should be either a solar or lunar eclipse. Can this figure be redrawn to show what actually happens when they line up? Why doesn't an eclipse happen every time they're in line?

An eclipse does happen every time they're in line. It's just that they're not in a line every month.

I took a look at the diagram, and I'm afraid that it can't be fixed that way. It's an illustration that is trying to explain the phases of the moon--if you were to use it as an accurate depiction of the Earth-moon-Sun system, you could also conclude that the moon had half the radius of the Earth (it has a quarter) and it was only four or five Earth radiuses away (it's actually sixty).

Also, the plane of the moon's orbit does appear to be tilted in the illustration, but it is probably more than is actual--and the orientation changes over the years, so that sometimes it lines up, and sometimes it does not.