Ask A Scientist© Physics Archive

Question:  Does the moon remain at a constant distance from earth? If so,
then why does the moon appear larger at the horizon, when it
would actually be further away from our viewpoint? And, if
the moon does _not_ keep a consistent distance from the
earth, then why does the moon appear so large at the horizon
anyway?
Thank you from Sonmeadow Home School.
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Answer 1:  Variations in distance too the Moon over a day are not
appreciable enough to cause any change in the size.  Moon
appears big near the horizon because you have things to
compare it with, like a distant building and so on.  Next time,
do an experiment.  Try to measure the angle subtended by
the Moon by keeping a coin or nail at an arms' length between
you and the Moon.  Measure the size when the Moon is near the horizon and
when it is high in the sky, nearer Zenith.  Compare two sizes.
Jasjeet
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Answer 2:  The apogee and perigee of the moon's orbit are 356400 and 406700 km
respectively, a variation of about 10% around the mean value of
384400 km.  Of course, it takes 2 weeks for the moon to move from
apogee to perigee.  (extra credit: why?)
John Hawley
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