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Why does the full moon appear to change size?

Why does the full moon appear to change size? It usually looks really big when it’s closer to the horizon and then smaller at higher elevations.
Katie Grist from Hertfordshire (age 15-24)


3 Responses

  1. The Moon’s orbit is elliptical and inclined relative to the earth’s equator. The point of closest approach to earth is called the perigee and is about 50,00Km closer than the furthest point from earth which is called the apogee. The image found at “Astronomy Picture of the day” for 25th October 2007 http://antwrp.gsfc.nasa.gov/apod/ap071025.html shows the difference very clearly.

  2. The fact that the full moon looks a lot bigger when it is near the horizon is known as the ‘Moon Illusion’ and has troubled people for hundreds of years! Most scientists agree this is an optical illusion

    See http://www.howstuffworks.com/question491.htm

  3. You can quickly and roughly check this for yourself by holding your thumb up to compare with the full moon on the horizon and also when it is directly above you. Within reasonable limits (discounting the changes described in the first answer) it is the same size all the time. When the full moon in on the horizon, you have other earth objects such as building to compare it with that contributes to the optical illusion that it appears to get bigger near the horizon.

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