THE Moon will play a spectacular trick tonight: it will appear extraordinarily large, like a big glowing ball hanging low in the sky.

This is the world’s largest optical illusion, known as the Moon Illusion and comes from tonight’s full Moon hanging at its lowest point in the sky than any full Moon since June 1987.

Part of the reason for the low full Moon is because it comes only a day after the summer solstice, the time of year when the Moon is at its lowest and Sun at its highest in the sky.

Added to that, this year also coincides near the peak of a 18.6-year cycle of the Moon as it circles slightly eccentrically around the Earth — what astronomers called a “lunar standstill”, which means that the Moon is even lower.

Robert Massey, an astronomer at the Royal Observatory, Greenwich, said: “There is something spectacular about this lunar standstill.

“It could be very nice with a good southern horizon, where you’ll see the Moon unusually large in the sky.”

Although the Moon appears large when it hugs the horizon this is a complete illusion — and here’s the proof.

When the Moon is on the horizon, hold out your arm and measure the Moon against your thumb. An hour or two later, when the Moon is slightly higher in the sky, it may seem to be smaller, but when you measure it against your thumb again it is exactly the same size. In fact, looking at the Moon through a cardboard tube will make the illusion vanish completely.

Why our eyes are tricked into believing that the Moon is larger than usual has been hotly debated since it was first written about 3,000 years ago by the Babylonians.

One reason, psychologists suggest, is that we see the sky as a flattened dome, with the zenith near and the horizon far away: birds flying overhead seem to be closer than birds flying on the horizon. Also, when the Moon is near the horizon, your brain miscalculates the Moon’s distance and size.

Another idea is that the brain interprets distances from clues on the horizon such as trees but is confused by greater distances, such as that between the Earth and the Moon, and over-compensates by making the Moon appear larger. Helen Ross, author of The Mystery of the Moon Illusion. says: “We judge size in relation to other sizes, but the apparent distance is much more difficult to understand.”

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