If you looked at the full Moon this week and thought that it looked surprisingly large, fear not, what you were seeing was the “Moon illusion”, a phenomenon known about for thousands of years. The Moon is still its usual size, but we think it looks so much larger because it is so low in the sky, skirting the horizon. The inflated size is all in our brains. We know that the Moon is its normal size in photographs, and if you view the Moon through, say, a cardboard tube it also looks perfectly ordinary.

There is some debate about what causes the illusion, though. One suggestion is that when the Moon hugs the horizon, objects in the foreground trick the brain into seeing the Moon as bigger than it really is. Another explanation is that we tend to misjudge distances in the sky – we see planes flying overhead as closer than planes on the horizon, for example – and so it is with the position and size of the Moon.

Whatever the explanation, this is the best time of year to see the Moon illusion as the summer solstice approaches. The Moon and Sun seesaw over the sky – when one is at its highest, the other is lowest. And at the summer solstice, the Sun reaches its highest point for the year, and the Moon is at it lowest.