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Name: Elham
Status: student
Grade: 12+
Location: Outside U.S.
Country: n/a
Date: Summer 2013


Question:
I was traveling with airplane at night, and I saw the moon below the aircraft! It seemed like we were in a higher place than the moon. I want to know the reason of this.


Replies:
Hi Elham,

Thanks for the question. Yes, the moon does "rise" and "set" in the night sky. If you have some time, you can watch the moon's movements in the night sky. This explanation could account for your observation. Alternatively, the aircraft you were flying on could have been in a turn (bank) and this may have resulted in an optical illusion. I should mention that optical illusions have been responsible for many aircraft crashes and for this reason, I stress that pilots should trust their instruments and not their senses.

I hope this helps. Thanks Jeff Grell


What probably happened was that the Moon was just rising in the east or setting in the west. If the airplane then turned one way or another, the Moon might appear to be below the plane. Good question.

David H. Levy


Elham

The moon is 238,900 miles (384,900 Kilo meters) above the Earth. The commercial airplane you are flying in is probably between 30,000 feet (9.144 Kilo meters) to 35,000 feet (10.688 Kilo meters) above the earth. Sometimes the moon can be seen to be beneath your airplane because your line-of-sight from the airplane to the moon is below the horizontal plane that the airplane is flying at. This can happen when the moon is rising from beyond the horizon or when it is setting below the horizon.

Sincere regards, Mike Stewart


Elham,

If you are on a hill or a mountain, or some tall building, and you happen to be looking down towards the base of that mountain or building, then whatever is attached to that base will appear lower than you. Now imagine that base extending outward, like a field or the sea. Everything attached to the edge of that field or sea would still appear lower than you. So if the Sun where to set (or rise) and touch the edge of that field or sea, then the Sun would appear lower than you. So, using the reference of a field or sea that is lower than your current location on top of a mountain or building, everything associated with the base would appear lower.

On the plane, the wing of the plane will appear a bit lower than you, and the clouds that happen to appear above the wing will appear higher. If the Moon appears lower than the wing or those clouds, then it will appear lower.

So, it seems that depending on what you associate as at your level (like the wing of the plane and some clouds) or below your level (the base of a mountain, the sea) then objects like the Sun and Moon can appear higher or lower than you.

Greg (Roberto Gregorius) Canisius College


This is an optical illusion, because it depends upon what your reference point is. Consider looking at the Moon while standing on the ground. It looks like it is ?up?. In fact we have incorporated this illusion into our language. ?The Moon is rising.? ?The Sun is setting.? ?The North Star does not ?move?.?

In reality we know none of the above is literally true. They all depend upon what our point of reference is.

Vince Calder


Hi Elham,

It has to do with the observer's position relative to the Moon's position over the horizon. The Moon was low over the horizon. Your plane was higher. The visual effect is that the Moon is lower than the plane.

This is much like the Moon very low over the horizon and a tree top appears higher than the Moon.

Peter E. Hughes, Ph.D. Milford, NH



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