There will be a total lunar eclipse this coming January 31. The celestial spectacle will be visible from anywhere in the Philippines roughly between 8:51 p.m. and 10:07 p.m., Philippine standard time.
During the eclipse, the Moon might turn reddish in color, which is why some use the term “blood moon” to describe the event. So why does the Moon turn red during a total lunar eclipse?
The short answer is this: Rayleigh scattering. However, it is worth looking into the longer answer because not only is it more informative, it is also quite beautiful.
First, let us review what lunar eclipses are. A lunar eclipse happens when the Earth is between the Sun and the Moon, so that the Earth’s shadow falls on the Moon.
When the Earth’s shadow covers only a part of the Moon, we observe a partial lunar eclipse. During a partial eclipse, we see the Earth’s round shadow on one side of the Moon. (The shadow is always round, evidence that the Earth is spherical.)
This Jan. 31, a partial eclipse will precede the total eclipse. The partial lunar eclipse will start at 7:48 p.m. As the eclipse progresses, the Earth’s shadow will cover more of the Moon.
By 8:51 p.m., the Earth’s shadow will fully envelope the Moon. If the Earth did not have an atmosphere, the Moon would be so dark by this time it will probably be invisible.
However, because of the air we breathe and the way sunlight interacts with this air, the Moon will turn into a dark shade of red. This will probably persist until around 10:07 p.m., when the total lunar eclipse will end and give way to a partial lunar eclipse, which will in turn later give way to a penumbral lunar eclipse.
The air we breathe turns the Moon red during a total solar eclipse because of the way it interacts with sunlight.
Sunlight, if you would recall your grade school science, contains the colors of the rainbow. These colors are red, orange, yellow, green, blue, indigo, and violet or ROYGBIV.
These colors are actually waves of light that our eyes detect and recognize. The colors differ from each other in the length of their waves. ROYGBIV arranges them in order of decreasing wavelength—red is the longest while violet is the shortest.
These different colors do not interact with the molecules of air in the same way. Instead, the colors with the shortest wavelengths, like violet, indigo, and blue, get scattered more easily. This is why our sky takes on a bluish color during the day. (It does not appear violet or indigo because of the way our eyes detect color.)
As sunlight passes through more particles in the atmosphere, more of the short wavelength colors get scattered away, leaving only the long wavelength colors, namely red and orange.
This is what happens during a sunset. When the Sun is about to set in the sky, the sunlight must pass through more particles in the atmosphere before it gets to you, so that by the time it reaches you, only red, orange, and a bit of yellow light have “survived.” When this light passes through even more atmosphere, even the yellow and orange can get scattered, leaving only red.
Aside from passing through the Earth’s atmosphere with minimal scattering, the red light coming from the Sun also experiences refraction. Refraction is the way light bends when it goes from one medium to another.
As sunlight moves from outer space to the atmosphere, it can bend toward the Earth. The sunlight that passes through the edge of the Earth bends another time when it moves back into outer space. This time, it bends toward the Earth’s shadow, bathing this shadow with the reddish color of sunset.
In fact, if you view a total lunar eclipse from the Moon, you will see a reddish ring around the Earth during the time that it blocks the Sun. This red-colored ring is a grand view of all the sunsets and sunrises happening on Earth at that time. During a total lunar eclipse, the Moon turns red because it bathes in the light of a million sunsets.
This is a truly astounding fact and one that makes our appreciation of an already spectacular event—the Moon turning red—even more powerful.
How red the Moon will turn during a given total lunar eclipse depends on the conditions of the atmosphere at the time. During an eclipse that followed the eruption of Mt. Pinatubo, there was so much dust in the atmosphere that the Moon was almost invisible during the total eclipse.
Whatever shade of red the Moon might take this Jan. 31, I hope we get to see it and appreciate the physics that connect us with our wonderful cosmos.
Pecier Decierdo is the resident physicist and astronomer of The Mind Museum.
