Giant Moon illusion tonight
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 18, 2008

Tonight the Moon will hover above the horizon, appearing roughly twice as large as it does when it reaches the highest point in our sky. We all know that the Moon doesn’t really change size, so let us remind ourselves how the Moon is tricking us, or rather, how our mind is deceiving us.

Left: Which line is bigger, A or B? The right hand image shows two red parallel lines joining the edges of the two yellow lines, proving that they are in fact the same length. This theory was originally devised by Mario Ponzo in 1913 and is sometimes referred to as the Ponzo Illusion. The same principle occurs for the so-called Moon illusion. Image: NASA/Tony Phillips.

Take a look at the drawing above (left). Which line is bigger, A or B? Although line A appears to be longer, if you measure the two lines you will see that they are the same size (right). Line A naturally appears bigger than line B because the rail road pattern disappears into the distance, which makes us think that the yellow lines are sitting at two different distances away from our eyes. This happens despite images of identical sizes converging on our retinas. Our collective minds are fooled into thinking that if object A is far away but is identical to the nearer object B, then A must really be larger than B.

So how does this explain the Moon illusion? Humans think of the sky as a flattened dome, which makes objects overhead seem closer than those on the horizon, because we are used to seeing clouds just a few kilometres above our heads when those on the horizon really can be hundreds of kilometres away. Even though the Moon is roughly 400,000 kilometres away, our minds think of it as being stuck on the inside edge of this relatively small flattened dome. Therefore we think that because the Moon is near the horizon it must be far away, but even though we know the Moon doesn’t really change size, we think it has to be bigger to make up for the 'extra' distance!

The flattened sky model for the Moon illusion. Regardless of its elevation, the distance between an observer at the centre of the horizontal line and the Moon remains constant, as shown by the unfilled circles. However, the Moon perceived as growing closer as its elevation increases must appear as growing smaller (filled circles). Graphic from 'Explaining the Moon Illusion' by L. Kaufman and J. Kaufman.
    
The Moon illusion arrives just a couple of days before the summer solstice, when the Sun is highest in the sky.

The Moon illusion can make a fun observation project for children (and curious grown-ups!). Check out some ideas for getting the better of the Moon illusion at www.starlight-news.co.uk/news.html

Moons captured on film appear the same size regardless of their elevation, as shown in this time-lapse sequence of images. The angular diameter of the Moon is always about 0.5 degrees. Image: NASA/S.Stephens.

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