The last total lunar eclipse of 2022 is expected on Tuesday. The next total eclipse won’t happen, NASA says, until 2025, while partial and penumbral lunar eclipses will still occur during that period. This eclipse is also a chance to witness the ice giant planet Uranus in the sky as the dim moon paves the way for other celestial bodies to shine brighter.
But why is the moon faintly red during an eclipse?
A lunar eclipse occurs when the Sun, Earth and Moon are aligned so that the Earth falls between them and the Moon briefly passes into the Earth’s shadow (Umbra, Penumbra). It is normally understood that the energy of the Sun that hits the Earth in the form of radiation is seen by the human eye as visible light that has wavelengths. Some colors have shorter wavelengths like blue and violet, while others like red have longer wavelengths. When the Sun is overhead, we see the sky as blue, but when it sets, we see that the sky is dominated by colors like yellow, orange, and red. This phenomenon is called Rayleigh scattering.
“The same phenomenon that makes our skies blue and our sunsets red causes the Moon to turn red during a lunar eclipse. It’s called Rayleigh scattering. Light travels in waves, and different colors of light have different physical properties. Blue light has a shorter wavelength and it is more easily scattered by particles in the Earth’s atmosphere than red light, which has a longer wavelength,” NASA explains in a post on its website.
The blue color, which travels in shorter, smaller wavelengths, makes it easier to be scattered by particles and dust, but during sunset sunlight hits us from the low sky and travels further before reaching our eyes. As a result, it passes through more air and particles that scatter and rescatter the blue light many times, in many directions. This removes it from the atmosphere and allows red, orange and yellow colors with longer wavelengths to pass through.
In the case of a lunar eclipse, when the Moon enters the Umbra (dark central part of the shadow), the path where all direct sunlight is blocked by the Earth – light from the Sun passes through the side of the Earth and is scattered by its atmosphere. By the time it reaches the Moon, only the longer wavelengths remain, which turn the Moon red. Once the Moon leaves the Umbra, it gradually resumes its original cycle and brightens as its usual self. A lunar eclipse happens about twice a year, and while you don’t need any special equipment to watch it, the best viewing experience comes from a dark environment away from bright lights.
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