What causes the illusion of two suns

Why is the moon so big on the horizon?

Everyone has been amazed: The full moon rises - and seems to us to be of unusual size. There are various explanations for this phenomenon. In fact, it is simply an optical illusion.

The satellite appears much larger on the horizon than when it is high in the night sky. An obvious physical reason could be that the light of the moon on the horizon has to travel a longer distance through the earthly atmosphere: The light is refracted in the air and this enlarges the image of the moon. Aristotle advocated this explanation over 2000 years ago and it is still circulating today - but it is wrong.

The moon

It is true that the moonlight on the horizon turns reddish due to the longer path through the atmosphere - just like sunlight does. However, it does not enlarge the moon. Rather, it is simply an optical illusion. Exact angle measurements show that the diameter of the moon on the horizon has the same value as elsewhere in the sky, namely about half a degree. The moon enlargement is not a physical phenomenon, but takes place in our brain.

The Ponzo Illusion?

One possible explanation for the apparent size of the moon on the horizon is the so-called Ponzo illusion, named after the Italian psychologist Mario Ponzo (1882–1960). Our brain constructs the size of an object by comparing it with other objects. For example, if you draw a horizontal line in two converging lines (like railroad tracks running to the horizon) at the top and bottom, the upper line appears longer to us, even if both are exactly the same length.

In the moon illusion, objects on the horizon - houses and trees - could play the role of rails. By comparing it with the objects, our brain constructs, according to the thesis, a larger moon than when the moon is lonely and alone in the sky.

Our image of the vault of heaven is flat

The flattened vault of heaven

But there is a problem with that explanation. Pilots and air passengers also report that the moon appears larger to them near the horizon - but from a great height there are no longer any comparable objects on the horizon. So there is no Ponzo illusion here.

That is why most researchers today assume a different explanation. People, it goes, do not perceive the heavens as a perfect hemisphere, but as a flattened dome. The zenith above us seems closer to us than the horizon. In fact, clouds, airplanes and birds justify this idea: in general, flying objects above us are actually closer to us than flying objects on the horizon. Our brain draws the reverse conclusion from this that an object on the horizon is further away and therefore actually larger than an object of apparently the same size above us. This means that our brain makes an object appear larger on the horizon than an object of the same size at the zenith.

New departure to the moon in research

Although the moon has been in the sights of astronomers for centuries and has already been visited by numerous space probes and astronauts, by no means all of the earth's satellite riddles have been solved. That is why the moon is currently increasingly becoming the focus of space research. The USA, the Europeans, China and India are planning new lunar probes - and at the end of the next decade, NASA plans to build a manned lunar station.