Ken Tapping, 25th September, 2013
On various solar science websites we can see movies of clouds of plasma and magnetic fields being ejected from the Sun. These clouds, called “coronal mass ejections”, can significantly impact us, which is why we have satellites watching out for them. However, a fascinating thing about these images is that we can see the stars too. Covering up the bright disc of the Sun with an opaque screen is enough to make the stars visible. On the surface of the Earth, blocking out the Sun with your thumb doesn’t do that. The glare into your eye is eliminated, but you still don’t see any stars.
The cause of the problem is also the reason we can live on the Earth. We have an atmosphere. Along with providing oxygen for us to breathe, it filters out damaging solar x-rays and ultraviolet radiation, moderates the temperature and provides a means of transporting water from sea to land. That atmosphere is loaded with molecules of nitrogen, oxygen and other gases, along with water vapour, water droplets, dust and pollutants.
These particles divert light by a process called Rayleigh scattering, named after the scientist who first explained it. Try this experiment. Get a clear glass jar or bottle, the bigger the better, and fill it with water. Drop in a piece of soap and shake it vigorously until the water goes foggy. This fog is due to lots of clusters of soap molecules floating in the water. Shake until the fog is too thick for you to see through the jar. Add soap if needed.
Set up the jar or bottle on a table in a darkened room, and shine a flashlight into one side of the jar. Look through the other side of the jar at the light coming from the flashlight. It looks orange or red, certainly much redder than the light going into the jar. Moreover the light coming from the sides of the jar is bluer than the light coming out of the end.
White light is a composite, made up of waves of different lengths. We see these wavelengths as colours, with blue light having about half the wavelength of red light. Some of the light passing through the soapy water scatters in all directions off the clusters of soap molecules. However, that scattering increases enormously as the wavelength decreases. Therefore it is mostly blue light that gets scattered out of the sides of the jar, leaving mostly red light to pass through to the other side. This property of scattering is used in a practical way. Wearing red or orange glasses improves visibility in haze or fog.
Beams of sunlight crossing the sky are subject to the same process. Blue light in beams of sunlight crossing the sky is scattered in our direction, making the sky look blue. When we look toward the Sun at sunset, which is analogous to looking at the direct beam from the flashlight, we are looking at what is left of the light beam after blue light has been scattered out of it. So we see the reds, yellows and golds that make sunsets so lovely.
Although this means there is usually no point in getting out the telescope until after sunset, this does not rule out all daylight astronomy. Infra-red light is scattered even less than red light, so from carefully chosen locations, such as the top of Mount Mauna Kea in Hawaii, or on the Atacama Plateau in Chile, we can do infrared astronomical observations during daylight. Radio waves are even longer, and are not significantly scattered at all, so we can do radio observations from locations near sea level, at day or night, in almost all weathers. It is intriguing to drive onto our observatory site here in BC during a winter snowstorm, and see the radio telescopes all busy observing happily. On the other hand, a sunset observed at radio wavelengths would not be very spectacular. Scattering can be a good thing.
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