Star light, star bright...

On the moon, the sky is black and if you shade your eyes from the bright sun and Earth, you can see stars during the day.

Here on Earth, it is sometimes possible to see bright planets such as Venus, and maybe a bright star or two during the day, if you know exactly where to look.

Apparently, it is easier to see stars during the day if you look up from the bottom of a deep well. I have not tried this and have no plans to any time soon.

We cannot see stars during the day from the surface of the Earth, but we can from the surface of the moon because the Earth has a nice, thick atmosphere and the moon has only an incredibly thin one.

The mechanism responsible is Rayleigh scattering, which is the scattering of light by very small particles floating in the atmosphere.

The wavelength — the distance between two wave crests — of light is very small, ranging from about 400 nanometres (billionths of a metre) for blue light and around 800 nanometres for red.

When sunlight light hits one of these particles as it passes through the atmosphere, it gets scattered. How much it is scattered depends on the wavelength of the light compared with the size of the particle, with the short wavelengths being scattered far more strongly than are the longer wavelengths.

This means a light beam passing through a layer of scattering particles will have its shorter wavelengths scattered off in all directions, so the beam emerging from the other side will be made of the longer wavelengths.

If you pass sunlight, or any other white light through a prism, or just look at a rainbow, where water droplets act as tiny prisms, the white light is shown to consist of light of all colours, added together.

A prism, or a raindrop, separates white light into its ingredients.

On a clear, sunny day, the sunlight passing across the sky encounters gas molecules in the air.

The size of the molecules is just right for scattering the blue part of the sunlight very strongly, while not significantly affecting the longer wavelengths.

Blue light gets scattered in our direction, making the sky glow blue.

If we get some bigger particles in the atmosphere, such as small water droplets, the scattering starts extending to the longer wavelengths, and the sky turns from blue to white.

At sunset, when we look in the direction of the sun, the sunlight is passing through a great thickness of atmosphere. The only unscattered wavelengths are the reds and golds, which makes sunsets so beautiful.

This raises an interesting question, why are sunrises generally not as spectacular? Years ago, I had an opportunity to see why in a graphic fashion.

In England, we were using a high-power radar on a 26-metre dish antenna to study clear air turbulence. This only reflects around a millionth of a millionth of the power hitting it, so a lot of power was needed in order to obtain a detectable echo.

In the morning, the atmosphere was generally clear of radar echoes, except from a thin layer maybe 10 kilometres up. Then, as the day progressed, the ground started to warm and the air in contact with the ground got warm too, and started to rise.

By noon, these bubbles were maybe two kilometres off the ground, and the bubbles were loaded with dust, insects and also birds feeding on them. By sunset all this stuff had climbed to around five km.

In the morning convection has not started, and the atmosphere is relatively clean of particles, so the rising sun is not reddened much. In the evening, after a day of convection, the light from the setting sun has to pass through all these additional particles, resulting in the glory of the sunset.

  • Venus shines brightly in the west after sunset.
  • Left to right: Mars, Saturn and Jupiter lie close together low in the southeast before dawn
  • The moon will be full on the 7th.


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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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