A baffling optical illusion will make the Moon appear unusually large during a partial lunar eclipse in the United States on Saturday. The eclipse, which will just over half of the Moon’s diameter obscured by the Earth’s shadow, will began at 10.17 GMT (3.17am PDT).
In many parts of the US and Canada, the eclipsing Moon will appear magnified as a result of an effect known as the Moon Illusion.
This occurs when the Moon is close to the horizon and the brain is somehow tricked into perceiving the Moon as larger than when it is higher in the sky.
Low hanging moons often appear unnaturally large when they are viewed through trees, gaps between buildings and other foreground subjects.
Measurements using cameras, however, have shown that a Moon low in the sky is no different in size from any other position in the sky and so the phenomenon is caused by a trick on the brain.
Nasa said that the effect will be particularly strong in western and central parts of the USA and Canada where the Moon will be setting as the eclipse reaches its maximum point – when 54% of the Moon will be obscured.
Because the Moon, Sun and Earth are not completely aligned, the eclipse will not totally obscure the Moon.
Observers in India, Japan and parts of East Asia will also experience the phenomenon.
Dr Tony Phillips, writing on the Nasa’s science news website, said: “A partial lunar eclipse is a beautiful thing all by itself.
“It almost makes you feel sorry for people living on the dreamy islands of the South Pacific. There the eclipse takes place directly overhead, high in the midnight sky where the Moon Illusion does not work.”