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< picture of the week >

 

 

variable moon

      Nope, this photo is not showing the Moon in zenith and on the horizon! Many people think that the Moon is bigger when it rises or sets, but it is only an optical illusion. Nature plays tricks with our mind - apparent size of the Moon doesn't change from the time it rises to the time it sets! Actually, the Moon is slightly bigger when itís low on the horizon, due to refraction. The precise reason for this optical illusion is not yet known to the scientists, but they guess that it has something to do with the larger number of "depth signs" that we can use when the Moon is low on the sky (houses, trees...), which allows us a more accurate 3D perception.

      So, what is that on this photo? Itís really interesting that although you will have really hard time trying to convince people that during one night the Moon doesn't change its size, a few notice that it changes in size significantly during one month! On your left you can see the Moon in apogee (the point on its orbit where itís farthest from the Earth). At that time, our satellite is 406 500 km away, with the apparent diameter of 29.5 arc minutes. On your right you can see the Moon in perigee, and when itís closest to us there is some 356 500 km between us, and its apparent size is around 33.6 arc minutes. As you can see, the difference is obvious! We can see a similar phenomenon with the Sun, which is smaller on our sky when the Earth is farthest from it (itís called aphelion, and it happens in the beggining of July) then when itís closest (on itís perihelion, around New Year).

Ana Brajoviś

 

 

 

 

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