MadSci Network: Astronomy

Re: Moon 'pointing' the wrong way illusion

Date: Sun Jun 6 06:15:09 1999
Posted By: Michael Martin-Smith, Other (pls. specify below), Family Physician, Fellow,BIS, amateur astronomer( BAA), British Interplanetary Society
Area of science: Astronomy
ID: 927901183.As

But the question is "How could one easily visualize what is going on? Could I set up a spotlight trained on a a basketball at night and move around the objects (or vice versa) in such a way that I could see the same effect? Any thoughts on how to explain this simply or visually?

I think this would be the logical way; a well collimated beam and a large dark beach ball would be needed- maybe attached to a long bamboo pole so that you could move it over a largish arc from behind a screen . Experiment would be needed to find the best size for the ball, and the most suitable beam width to show it well.

[Moderator's note: One of the neat aspects about the phases of the Moon and the appearance of the Moon from Earth is that they can be demonstrated with simple equipment. One just needs a bright light to represent the Sun (it doesn't have to be collimated as the Sun itself doesn't produce a collimated beam), a ball to represent the Moon, and a darkened room. By moving the ball (Moon) around your head (Earth), you can see the phases of the Moon. One thing to keep in mind while doing this experiment is that the Moon does not orbit the Earth in the same plane that the Earth orbits the Sun. Rather, the Moon's orbit is inclined at about 5 degrees to the Earth's orbit. That's why we don't have a solar and lunar eclipse every month.]

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