Why is the sky blue?

By Fred Watson 6 January 2010
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Astronomer, award-winning science communicator and AG columnist Fred Watson tackles the big questions about the Earth, Moon and other heavenly objects.

Why is the sky blue?

Poor old Leonardo da Vinci. As an artist who dealt in light, colour and movement, he wanted to understand how nature produces all these phenomena. And he had a jolly good shot at it, from analysing the tumbling of water over stones in a brook to reasoning why the faintly illuminated lunar disc can sometimes be seen between the horns of the crescent Moon – “the old Moon in the arms of the new”. Thus Leonardo might be described as the first real scientist; certainly he was one of the most accomplished figures of the Renaissance.

One thing eluded him, however. Why is the sky blue? He got close to the answer in a manuscript he penned in 1509 (which is owned today by Bill and Melinda Gates, of Microsoft fame). Here, Leonardo explains that he thinks the blue of the sky is caused by the mixture of white light – from sunlight illuminating the air – and the blackness of space beyond. A good idea, but wait a minute – shouldn’t that produce grey? And a fairly dirty grey, at that?

While he was on the right track, Leonardo wasn’t equipped to understand the essential ingredient, which is a subtle optical phenomenon called scattering. Indeed, it was not until 1871 that the noble English scientist, Lord Rayleigh, got to the bottom of it all. Scattering is something that happens whenever light rays strike microscopic particles – including molecules of air. Thus, the flood of sunlight hitting the atmosphere is simply scattered in all directions, which is why the sky is luminous in daytime.

The reason it is blue, though, is that Rayleigh’s theory predicts that blue light is about 3.2 times more likely to be scattered than red light in any interaction with a particle. And that is exactly what we see. While the light of the sky still contains red light, it is overwhelmed by the blue.

Why do stars twinkle?

Once again, scientists have a fancy name for a common phenomenon that everyone is familiar with. It’s scintillation and, in this context, it refers to the effect that our continuously moving atmosphere has on light.

If the atmosphere was at exactly the same temperature throughout, the stars wouldn’t twinkle and the night sky would lose much of its charm. In reality, however, the atmosphere is made up of countless bubbles of warmer and cooler air, jostling together as they are carried along by the wind. This turbulent motion takes place at all heights above the ground, and is often very rapid in the high-altitude jet stream.

Because the temperature of air governs its density – and that, in turn, dictates the extent to which it will bend light – these moving atmospheric cells act like invisible lenses. Admittedly they are very weak lenses, because the refractive (light-bending) power of air is feeble, but they are enough to cause the twinkling of stars – and to compromise observations made with giant telescopes.

Twinkling arises because the moving blobs of air act to alternately focus and defocus the starlight arriving at our eyes. The effect is that the star gets rapidly brighter and dimmer – that is, it twinkles, or scintillates. Looking through a telescope, you can detect this focusing and defocusing as the star image seems to contract and expand. It is also in frenzied random motion, like a moth fluttering around a lamp, as the light rays are bent this way and that in the moving air.

For 300 years, astronomers have referred to the degree of turbulence in the atmosphere as “seeing”. When the seeing is good, the star images in their telescopes are stable points of light. When it’s not, they become inflated, trembling blobs, and the exquisite detail that the telescope is capable of revealing is lost altogether in the shaking of the air.

Generally, the seeing gets better as you look higher in the sky – and thus the twinkling seen with the unaided eye is reduced. That is because the path of starlight through the atmosphere is shortest for stars that are overhead. For the same reason, high-altitude sites like mountain tops have better image quality for astronomers than sea-level locations – there is less turbulent air above them. The seeing is also strongly dependent on local weather conditions. It is often depressingly poor after the passage of a cold front, for example. But the twinkling on such occasions is, well…charming.

Why does the Moon look so big when it’s low in the sky?

Of all the phenomena relating to the Moon, this one is undoubtedly the most frequently remarked upon. It is especially obvious around the time of full Moon, when the disc of our satellite looks positively huge as it rises at dusk in the eastern half of the sky. And the full Moon appears similarly enormous when it sets towards the west as dawn breaks. The effect is extremely convincing – psychologists tell us that the Moon seems to be two to three times larger near the horizon than when it is high in the sky. You’ll have noticed I said “psychologists” here, and not “physicists”. That is because this effect is not real, but takes place entirely inside our own heads.

It is the human brain that causes the apparent increase in size, not the physics of the atmosphere or the geometry of the Earth-Moon system. The effect is known famously as the ‘Moon illusion’.

There are many simple ways of proving that it is, indeed, a psychological illusion. A thin pencil held at arm’s length, for example, will just about cover the Moon whether it’s high in the sky or low on the horizon. And photographs of the Moon taken in the two situations prove the same thing.

So why do our eyes tell us it looks so big? This conundrum of perception has long been a subject of debate among cognitive psychologists. There is at least one hefty volume of learned text on the topic, written by some of the world’s foremost experts during the late 1980s. Browsing through this tome gives the distinct impression that psychologists were (and perhaps still are) groping to understand why our minds tell us so convincingly that the Moon is bigger than usual when it’s simply not true.

It used to be thought that the Moon illusion arises because a low Moon is surrounded by familiar objects like hills, trees and buildings, while a high one is seen in isolation. Thus the horizontal Moon is seen at its “real” size, but the lonely Moon high in the sky appears diminished. There is nothing familiar to compare it with, so it shrinks. More recent thinking suggests there’s more to it than this, however, and the most popular of today’s theories relates our impression of the size of the Moon to the perceived shape of the sky itself.

It has been known for many years that we think of the sky not as a hemispheric dome over our heads, but as a flattened dish. In other words, our perception of the sky is that it is much further away near the horizon than over our heads. It’s no mystery why this should be so – those fluffy clouds we often see scattered uniformly over a fair-weather sky, for example, appear to become smaller as our gaze moves down from the point overhead (the zenith) towards the horizon. Of course, in reality, they’re all similarly sized, but the effect of perspective makes them seem to shrink. On a very clear day in flat country, where the view is uncluttered by trees and buildings, clouds near the horizon appear perhaps 50 times smaller than their local counterparts because they are so much further away. No wonder our brains tell us that the sky is more like an upside-down dinner-plate than an upside-down fruit bowl.

Now, into this trick of perception steps the Moon.

We most often see it high up, so we have a built-in impression of how big it should look. As it rises over the horizon it is, as we have seen, essentially the same size as it always is. But hang on a minute – the human mind says that anything near the horizon is a long way off, and therefore must look small. The Moon, however, is the same size, challenging the brain to compute what is going on. How does the brain respond? There are no prizes for guessing that it tells us the Moon is bigger than it should be, because it doesn’t look as small as the brain expects. Hence the Moon illusion.

Finally, many commentators have noted that the Moon illusion can be made to disappear by standing with your feet astride and bending down to look at the Moon between your legs. Personally, I have always thought this was a cunning ploy on the part of astronomers to make the rest of us look stupid, but there is probably something to be said for radically changing the way you observe the scene to dispel the deception. A bit more speculatively (since there’s clearly no need to adjust your nether garments to observe the Moon), I wonder if this could be where the expression “mooning” comes from?

Source: Australian Geographic Oct – Dec 2007