Or, for that matter, a solar eclipse? If the moon is going around the earth once a month shouldn’t the moon’s shadow fall on the earth (a solar eclipse) every month, and the earth’s shadow fall on the moon (a lunar eclipse) once a month?

Yes, it should,and all the planets and moons and stuff should all be on the same flat plane with the sun in the “middle.” Someday astronomers will find a solar system with several planets and they’ll name it something special because it will be very rare.

Anyway, this came up in conversation and the conversation led to some googling around and this nice video that explains it all in very simple terms came up:


  1. #1 Dan Milton
    September 24, 2011

    I once helped a junior college instructor grade an exam that asked that question. My favorite answer, “A lunar eclipse can only occur during a full moon and there might not be a full moon that month”.

  2. #2 Greg Laden
    September 24, 2011

    That’s a great answer because it is wrong TWICE! Technically, there is not a “full moon” during total eclipse months.

  3. #3 wfr
    September 24, 2011

    So what is the path of the moon around the sun? Epicycloid? Hypercycloid? What?

  4. #4 Greg Laden
    September 24, 2011

    It is a mere wiggly ellipse. The moon always goes in the same direction around the sun (never backwards). As does the earth. Both of them have a wiggly path because of each other, but they are both just going around the sun.

  5. #5 yourong
    September 25, 2011

    you ask why elipses dont happen twice a month. simple answer every thing in space is moving forwards

  6. #6 Greg Laden
    September 25, 2011

    No, that is not the reason. You are wrong. Consider reading the post.