Why does Moon on the horizon look bigger than when it is high in the sky?

To stand on the seashore or an open plain and watch the huge globe of the moon appear majestically over over the horizon is a commonplace but wonderful experience. Strangely, the moon's orb looks enormous, and it appears to rise quite fast, like a huge gas balloon ascending into the heavens. A short time later, when the moon is higher in the sky, it seems to be much smaller, and it may be difficult to detect that it is still rising.

The moon's size and speed are constant. It moves, in relation to us, at 97 200 km/h and it an orbit that keeps it about 384 400 km from the Earth. Any perceived change in it size or speed in a stick of the eye, an optical illusion. It is not caused, as some people believe, by a form of atmospheric distortion that magnifies the Moon or speeds it up. In fact, atmospheric effects cause the Moon's measured diameter at the horizon to be slightly smaller than when it measured higher in the sky.

The illusion is caused by the way our brain interprets information. A rising Moon is placed on the horizon, and perceived as being more distant than one immediately overhead. Nearly two centuries ago, English mathematician and astronomer Robert Smith concluded that at the horizon the sky appear to be three times as far away as it does at its zenith overhead. This is because land-based object, such as tree and buildings, lead us into a wrongful comparison.

The same trick makes the Moon look bigger. When it is high in the sky, we have nothing to compare it with. Similarly, we think that it is rising faster, because we can see the gap between it and the horizon constantly widening. Later, the gap is too vast for us to detect any movement of the Moon at all.

Another curious example of our brain leading us into mistake conclusion may come when viewing the moon through binocular. Many people claim that the naked eye, the reverse of what is seen when looking at other objects through binoculars. The explanation for this is probably that when we use binoculars other object are excluded, thus removing any basis for comparison.

If u want to see the Moon shrink suddenly in size, try this: hold up a peat arm’s length, and compare it with the Moon. You may be surprised to find that the Moon looks about half the size of an everage pea.