MadSci Network: Astronomy

Subject: Moon 'pointing' the wrong way illusion

Date: Fri May 28 09:19:43 1999
Posted by Rocky Bernstein
Grade level: No grade entered. School: No school entered.
City: NY State/Province: NY Country: USA
Area of science: Astronomy
ID: 927901183.As

When the moon is 1/4 full or less sometimes you can see both the
sun and the moon in the same sky. 

There is this illusion that although the moon roughly "points"
to the sun it doesn't point directly to it.

By "pointing" to the sun I mean if you draw a line from the midpoints
of the outer and inner visible arcs of the moon and extend that,
simplistically one would expect that line extended in the direction of the
sun would touch the sun.

Since the moon moves about 180 degrees or so in the sky in a night while
the phase of the moon changes only 1/30th or so, it would be impossible
to expect the moon to "point" to the sun the entire time. 

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? 

Re: Moon 'pointing' the wrong way illusion

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