Size of the Moon Question

Question

Let’s say you were to do an experiment using a coin that helps to find the distance from earth to moon through measuring the distance between your eye and the moon with the coin. This will later determine the diameter of the moon.

How will the result of the diameter of the moon change if the moon is at the overhead and if it’s at the horizon?

Will the calculated diameter result be bigger if it’s at the overhead or the horizon?

Answer 1

Hi Soph L.,

I hope that you have deduced that you use the coin (held flat across your line of sight) to guage the relative diameter of the moon, and not that you attempt to flip the coin end-to-end through space, for 285,000 miles, to measure the distance directly!

So, the subtended angle of the moon disc doesn't change between the horizon and the zenith. And the coin will therefore measure it the same in either position. This contradicts our apparent visual impression that the moon is much larger and dimmer near the horizon. But that apparent visual feeling, termed "The Moon Illusion", is very convincing!

There is a minor change in the subtended angle of the Moon due to its slightly elliptical orbit around the Earth. But that's quite small, maybe 10%. You wouldn't notice that by eye, though it would make a difference in the duration of totality during a solar total eclipse.

-- Cheers, -- Mr. d.