If you have ever seen a lunar eclipse, you have noticed that the Moon turns red when it gets into Earth’s shadow. It may sound strange, but this spooky transformation is not a sign of doomsday but is caused by the atmosphere of our Earth.
The reason for this color change is a phenomenon known as Rayleigh scattering. 19th-century physicist Lord Rayleigh (John William Strutt) discovered that extremely small particles that are present in transparent solids, liquids, or gases are capable of scattering electromagnetic waves, including light.
He also determined that the strength of light scattering is inversely proportional to the fourth power of the wavelength, which means this effect is much more pronounced for the shortest electromagnetic waves.
What causes the Rayleigh scattering?
This phenomenon is caused by small particles that are present in our planet’s atmosphere. These particles must be small for the effect to occur – in fact, they need to be about one-tenth of the light wavelength or smaller, down to the size of molecules or individual atoms.
The waves from the blue part of the solar spectrum get scattered much more strongly than red waves. Blue (shorter) waves get scattered to all sides, while much of the red (longer) waves remain intact and propagate along the original trajectory.
How does Earth’s atmosphere make the Moon turn red during an eclipse?
During a lunar eclipse, the Sun, Earth, and Moon are aligned, and Earth is between the other two. When a total eclipse occurs, the entire Moon falls within the darkest part of Earth’s shadow, called the umbra.
When the Moon is within the umbra, it turns reddish. The light rays bend around the edges of our planet before reaching the Moon. The red hue on the Moon’s face is visible even during a partial lunar eclipse.
Due to the Rayleigh scattering, the shorter-wavelength blue light is scattered the most when the light passes through Earth’s atmosphere. Therefore, only the orange and red, which have longer wavelengths, reach the Moon. That’s why you see the Moon in a red hue.
Moon turns red because of the same reason that makes our sky blue
Even though it may sound strange, the same phenomenon explains why the sky here on Earth is blue. The longer wavelengths, such as reds, oranges, and yellows, pass the atmosphere while violets and blues get scattered by particles and dust that are suspended in different layers of air.
During the sunset, however, the sky may appear reddish. When the Sun is closer to the horizon, the light has to travel a longer distance through the atmosphere, which means it must pass a thicker environment to reach the observer, encountering more light-scattering particles on its way. Therefore, shorter wavelengths are scattered too many times, and only longer wavelengths, such as red, are visible to our eyes.