The Moon illusion is famous and intriguing phenomenon, the rising or setting of an unbelievably big full moon. When the full moon rises on 6 April around 8pm BST and you are able to get to a location that has a clear east-south-east horizon so as to see the moonrise in its earlier stages, then you can experience this illusion first hand. The phrase 'you can't believe your eyes' is often banded about, but this time it's true. And you are not alone as explanations for this phenomenon have been sought since ancient times and surely recognised since humans first gazed in awe at our beautiful companion in space.

The 'Moon illusion' as the full moon sets over the VLT telescopes in Chile. Image: G. Gillet / ESO

It's all the fault of your brain deceiving your eyes. The perception is that the full moon looks much bigger at moonrise than when it's nearer the zenith. Distant buildings, hills or trees in the line of sight heighten the effect. It takes much convincing that this is not the case but it's an illusion caused by us perceiving that the shape of the sky is a 'squashed dome rather than a high hemisphere', as Astronomy Now's lunar expert Peter Grego puts it. The same illusion occurs with the rising and setting of the Sun through haze at the horizon. Of course the Moon doesn't always have the same angular size in the sky due to its elliptical orbit around the Earth; This month it's closest to us (perigee) on 7 April at 6pm, when it's as close as 358,311 kilometres and at its farthest from us on 22 April (apogee), at a distance of 406,421 kilometres. The effect of this is to make the Moon almost four arcminutes larger at perigee and the fact that the full moon and perigee are but a day out from coinciding this month, reinforces the Moon illusion.

So if your local forecast promises a clear early evening then why not try to see for yourself this almost wondrous sight; children especially are sure to be wowed. But this is not the end of it as May and June's full moon will be belters too, especially the latter. For more on the Moon illusion pick up a copy of the current (April) edition of Astronomy Now. If you manage to get any nice pictures we would be delighted to receive them at Astronomy Now. Send them to gallery2012 @ astronomynow.com