I am a keen sky-watcher but sometimes I just have to gaze in awe at the moon – it looks absolutely massive!  This usually happens when it is low on the horizon. Obviously the moon hasn’t suddenly changed size, it’s still 3,476km in diameter. So perhaps it has moved nearer to Earth?

The moon’s orbit around Earth is elliptical and at it’s closest point (the perigee) it’s 363,104km away. At the moons furthest point (the apogee) it’s 405,696km away – a difference of 42,592km distance. Not surprisingly this does lead to a difference in observable apparent size.

However to see the maximum difference you would need to see the Full Moon at Perigee and compare it with the Full Moon at Apogee and that happens six months apart! This small difference does not account for the Moon sometimes appearing absolutely massive - so what is happening?

To understand this, have a look at the diagram. The straight lines are exactly the same length and the orange circles are exactly the same size.

If you don’t believe me get a ruler and measure them! They look different sizes don’t they? This is known as the Ebbinghaus Illusion.

Now consider a cloudy sky. When you look directly up, the clouds look bigger than those on the horizon. Instead of all parts of the sky looking the same distance from us, the higher up we look the closer it seems, almost as if the sky is squashed down on us.

When we judge size against distance we subconsciously use a ’size constancy’ effect.  Our minds adjust for size and distance all the time, and this adjustment can make the moon look massive. I will explain how tomorrow. 

John McNally