Blue Skies, Red Sunsets.

"Hmm, nice colour. Where are we now Albert?"

We're entering the bottom layer of the atmosphere called the troposphere. This is the part of the atmosphere that you would normally think of as the sky, the part that is breathable and is affected by the weather. This nice blue is the colour of the sky seen from the Earth.

"Why is it blue?"

Because of the sunlight passing through it.

"But sunlight isn't blue."

No, but as Isaac Newton showed with his prisms, sunlight contains all colours of light including blue. As sunlight passes through the atmosphere the blue light is much more likely to be knocked off course by gases in the atmosphere like oxygen and nitrogen. It is this scattering of blue light, called Rayleigh scattering, that makes the sky look blue.

"Hmmm"

I see I'll have to try a bit harder than that. Now consider all the light coming from the Sun.

"Okay."

Now most of the light goes in a straight line so that on a clear day you can see the Sun.

"So far so good."

The light from Sun that makes the sun look like, well… the sun, is heading straight towards you. But there is much more light that misses your eyes and speeds on through the atmosphere. Now, the light that isn't going towards you should be invisible because it can't reach your eyes. However, a small fraction of this light is visible because the sun's rays are knocked off course by air molecules. Because of their size, air molecules scatter ten times more blue light than red. So wherever you look in the sky your eyes will be hit by some of this scattered mostly blue light that comes from sunlight passing through the atmosphere.

“I thought it was something to do with light reflected off the sea?”

It’s nothing to do with light from sea. The sky looks the same colour in the middle of an ocean or in the middle of the Sahara desert. This same effect is why the Sun looks redder at sunrise and sunset. When the Sun is near the horizon, the rays of light have to pass through more of the atmosphere.

"So more of the blue is scattered leaving yellow and red light."

Very good. The light that is left instead of being yellowy-white becomes red because the blue is missing. It goes back to Newton and white light being a mixture of all the colours of a rainbow. Subtract a colour and what's left isn't white any more.

"So what's that white thing then?"

It's a cloud.

"We went past loads of clouds of dust and gas in space and none them looked like that."

That’s an Earth cloud. It's made up lots of tiny water droplets. Air contains a lot of water as a colourless gas known as water vapour. This is what is meant by humidity. If there is too much water or the temperature drops then this water stops being a gas and becomes droplets of a fine liquid. If it happens on the ground it's called dew. Just above the ground it's called fog. At twenty thousand feet it's called a cloud.

"So why isn't it blue like the sky?"

The water droplets are much much bigger than the air molecules that make the sky blue. Big things like water droplets scatter light of all wavelengths completely equally. So the light that you see scattered from a cloud is white because it contains an equal mixture of all the colours contained in sunlight. If clouds are very thick then not much light gets through at all, so rather than looking a nice bright white colour they look grey. From white to black via grey just involves less and less light. In light terms, grey is just a dim white. Clouds can, of course, be pink at sunrise and sunsets. Clouds can only scatter what reaches them. At sunrise and sunset the blue light in the sun's rays have all been scattered so all the clouds can do is scatter what is left, and that's pinky red light.

"One last question."

Fire away.

"Where do we fit in? I mean the sky's blue so it must be daytime, but stars disappear in the daytime, so how can we be here at all?"

Just because something can't be seen doesn't mean that it doesn't exist. We're here along with all the rest of the photons from stars and other galaxies. It's just that we are outnumbered by all the scattered light from the sun. On those very rare occasions when the moon happens to block out the sun, in a total eclipse of the sun, suddenly all the stars will appear in the middle of the day. All the stars you can see in the sky at night during the winter are up there in blue sky during the day in summer. A star like ours which is in the night sky in summer is up there during the day in winter. So stars don't come out at night, they just become more visible. If you watch carefully as the sun sets on a clear day you start to see first the very brightest few stars and then as it gets darker slowly all the other stars start to be visible or in other words stand out from the background.

"But we're invisible, and you said we were going to be seen."

We can be seen just as easily as any other photon. It's just that eyes can't pick out where we've come from in the middle of the day because of all the other photons. We're not invisible, it's just our star that's invisible at the moment. Every scene needs almost uncountable numbers of photons to be seen. So every photon makes its own very tiny contribution.

“So I’m tiny and insignificant?”

No just tiny, and you’ve seen more of the galaxy than any of those earthlings down there so I think that makes you pretty significant.

"Wait a minute. Look ahead, look ahead, it's a black hole."

I think that’s a bit unlikely in someone's back garden.

"But it's round, black, straight ahead and we're about to hit it."

It is also flat, a tenth of a millimetre across and has no gravitational field.

"So what, it's black, that means there is no light coming out of it. That means we're about to disappear."

Perhaps.

"Perhaps? Is that all you can say Albert!"