Spacetime Sunday: Twinkle, Twinkle, Little Star

Have you ever been swimming, gone to the bottom of the pool and looked up through the water, seen the lights dance?
Even though you know the light is staying still, to you it appears to occupy much more space than it actually does, blurring its true position.

That’s similar to what is happening. But instead of water and the refraction that is occurring, it is a fluid of another kind; our atmosphere. Every second we’re just going about our business at or near sea level, 10,000 Pascals of pressure push down on us- that’s about 14 lbs per square inch! There’s a lot between here and space. With that much gas and dust to get through, light travelling to us is bound to be affected. The turbulent mixing of the atmosphere causes distortions of the the light travelling through, and it is always on the move, so images received on earth are constantly changing.

http://chemwiki.ucdavis.edu/@api/deki/files/816/atmosphere-nasa.gov.jpg

In reality, anything we look up at in the sky is twinkling in this way, but the planets, the sun and the moon are all much closer and appear much larger in the sky, so rather than one point of light, they look more like disks on the sky (if only slightly more in the case of planets). So although it is still happening, the effect is more subtle with objects closer to us, because the area of the light source appears bigger on the sky, so even if individual light rays are subject to this, they’re still more likely to appear at our eyes inside the apparent disk the planet makes on the sky.

If you look at the moon with a telescope you start to see these effects. In fact when it comes to taking photos from the ground, it is really unhelpful to have an atmosphere (even if it helps us live to take the photo)! Besides having telescopes in space to remove the problem (which is wildly expensive), optical astronomers have their telescopes in high altitude places where the effects of this are lessened. They also have many a trick up their sleeves to cancel this effect, my favourite being adaptive optics, where a laser is pointed into the sky to make a pretend star and then since it is known what the laser point should look like, they can effectively see how the atmosphere is behaving, and using fancy mirrors inside the telescope, cancel it out. This is much cheaper than launching satellites, plus you get to work on top of a mountain! This effect is known as ‘seeing’ in astronomy. The better the seeing, the less distortion there is of the image.

The Very Large Telescope in Chile uses adaptive optics

It is not just imaging distant stars that this technique improves, either. Medical imaging of the eye benefited from this technique too! Often the things astrophysicists invent end up having incredible uses down on earth, too. More on that later in this blog feature when I talk about why we need space programs. Next week we’ll look at the treasure chest that is Orion, and you’ll never look at him the same way again!

Spacetime Sunday: Why is the sky blue?

When the Sun or Moon are low in the sky, they look orange or red. When they’re overhead they look yellow or white. But why?

Essentially, its a bit like what makes rainbows. White light is bent inside a raindrop, and is split up upon exiting. The similarity is that because light contains lots of waves of different wavelengths, (each wavelength corresponding to a different colour), these waves have different properties. One of which is how much waves are bent by refraction and scattering.

 

So in the rainbow case, the white line is sunlight, the prism is the raindrop, and the split spectrum is the rainbow. There has to be a specific angle the white light is coming in at for this to happen (the answer to life, the universe and everything, 42 degrees). This is refraction. It happens because the air outside and the raindrop inside have different extents to which the interfere with the straight line of the light ray, causing them to travel differently inside and outside.

The waves of light in the blue wavelengths are bent through a greater angle (2) than the angle of refraction of waves corresponding to red (1)

In the case of the sky, the white line is again sunlight, but this time, rather than being bent, the light is bouncing off the molecules like dust and air of our atmosphere, and this is scattering. Like we said before, different wavelengths behave differently in these events. You can see in the picture above, that the change in direction of the blue light after refraction is a lot more than the change in direction of the red light. The same is true for scattering, the blue light scatters a lot more than the red, spreading out across the sky, and along with humans not having many purple receptors in our eyes, the dominant colour in the sky is blue.

 

What about sunset then?

As the sun’s apparent position in the sky gets closer to the horizon, the more atmosphere the light has to pass through to get to us and that means more scattering, effectively filtering out the blue from the path of the sun’s rays. As the evening progresses, the sun will look like it is changing colour from yellow, to orange, to a stunning red, as the thickness of the atmosphere the light has to travel through increases and that means even more scattering of the blue light, allowing more of the reds than blue travelling straight to your eyes  (though we wouldn’t look straight at the sun, would we!)

This is why sometimes cities have even better sunsets than somewhere with clear, unpolluted skies.

A simple experiment, to demonstrate much clearer than I can explain, is in this video. Start at 6:00 to see the experiment Tyndall did in the 1800’s recreated and explained by Prof Brian Cox, or better, watch the whole thing.

 

So next time someone answers a question with “Oh I don’t know, why is the sky blue?!” You can tell them exactly why!