Blue Sky, Blood Moon

A Lunar Eclipse producing a blood Moon. Credit: instagram: @anthonyduffphotography, as cited by ABC

Blue sky – a familiar sight.

Blue ocean – a sight to see.

Red moon – a sight to behold.

Let us ponder a few simple questions. Why is the water clear in our sink while blue in the ocean? Why is our sky blue during the day, yet at sunset, our eyes are greeted with a peaceful pink. Our Moon, a gentle giant in the night is often a bright white. Why then, does it bleed every so often? 

All of these phenomena are the result of the same physical process, the scattering of light. White light that comes from our sun contains all colours of the rainbow. In fact, a rainbow is formed when light from the sun is scattered by water in the sky.

Rayleigh Scattering (and Tyndall Scattering)

The different colours of light represent how much energy the light has. Blue light has more energy than red light (see Light) and as a result, is more likely to interact with the particles in our atmosphere. One way to understand this phenomenon is to think of light in terms of dogs. A puppy has lots of energy and is more likely to run into things whereas, an older dog has less energy and doesn’t cause as much havoc. 

As the blue light is more likely to run into atoms or groups of atoms (molecules) in our atmosphere, it scatters more easily. The blue light scatters throughout the sky whereas the red light passes through. This process is why the sky is blue, and is known as Rayleigh scattering. Tyndall scattering is a similar process but refers to scattering due to larger particles such as dust or smoke. 

During a sunset, light travels through more of the atmosphere. By the time it reaches us, most of the blue light has been scattered, leaving only the red and orange light. This is also why the sun appears red on the horizon, and white in the sky. (DO NOT LOOK DIRECTLY AT THE SUN)

sky-diagram
A summary of how light travels through the atmosphere. Credit: ask a scientist

The Moon and Ocean

This scattering also occurs in large bodies of water, such as the ocean. The blue light interacts with the water more readily than red light and as such is reflected. 

The blood moon is a more interesting case but follows the same process. A blood moon occurs during a lunar eclipse, when Earth passes between the Moon and the Sun. This means that light passes through Earth’s atmosphere before it reaches the Moon. Since most of blue light is scattered in our atmosphere, only red light remains (just like a sunset). 

lunar-eclipse-diagram.jpg
A Lunar Eclipse results in a red, or blood, Moon. Credit: Pacific Science Center

Do It Yourself

You can test this idea yourself at home. All you need is a large clear container, water, a torch and a few drops of milk. Add the milk and water into the container and mix. The milk represents the particles in our atmosphere. Shine the torch so that light passes through the container as shown below. 

The “sunset in a box” experiment.

As you can see in the image above, light passes from right to left. Initially, the beam is white. As it travels through the milky water, it scatters until the beam is no longer defined. Towards the left, the light has changed to  an orange/red colour, just like a sunset. 

Conclusions

Rayleigh Scattering is the process where light scatters by interacting with particles. Since blue light is more energetic than red, it scatters through the atmosphere to create a blue sky. This same process produces blood moons and blue oceans. 

References and Resources

BBC Earth Lab with Brian Cox

Encyclopedia Britannica: Rayleigh Scattering and Tyndall Effect

Edited by Justin Lillecrapp

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