The reflection of the sunlight is what makes it possible for us to contemplate the Moon with a brilliant hue, in all its splendor. We can see the moon in small, large or even during all its phases. In spite of everything, sometimes, we can see it in red during lunar eclipses. When this happens, it is known as the Blood Moon or Red Moon, a very strange and curious phenomenon.
The lunar eclipse is a very strange and interesting phenomenon to contemplate. This phenomenon occurs as a consequence of the Moon passing behind the Earth’s shadow and darkening.
It can be divided in the shadow of the Earth into two parts: the umbra and the penumbra. The penumbra receives a minimal amount of the Sun’s rays, while the umbra receives none. Precisely, a lunar eclipse is related to the passage of the Moon through the umbra. Despite not receiving any ray of light, the umbra is not completely dark, because the Earth’s atmosphere redirects the Sun’s rays and lets certain wavelengths pass through, while others scatter them.
Why the Moon Looks Red in a Blood Moon Eclipse
As we said, during a lunar eclipse the Moon passes behind the Earth’s shadow, but it does not look completely dark. This is because the gas molecules that make up the Earth’s atmosphere scatter the wavelengths that belong to blue and green light, but directly pass those that are more reddish. This phenomenon is called Rayleigh effect and it is the reason why the Moon when passing through the shadow of the Earth, will receive the red glow of the atmosphere . Sunlight that reaches the Earth’s atmosphere, passes through it and bounces off the ground towards the Moon. In an eclipse, in addition to this, blue light is filtered as it passes through the atmosphere and the Moon can be seen in red . This is the Moon that we call the Blood Moon.
During the day, sunlight in the reddish spectrum passes through the atmosphere and reaches the ground, while the blue spectrum is scattered in all directions. This also explains why the sky is blue and sunsets and sunrises are reddish.
But the red Moon is not exclusive to eclipses, we can also observe it when the Moon rises at night or sets at dawn . This is produced by the same phenomenon that we discussed for lunar eclipses: that of the dispersion of certain wavelengths.
Another reason that can explain this phenomenon is air particles, as in the case of a volcano eruption or a large fire. These are phenomena that eject particles, obscuring the surrounding light except for red light.
Although lunar eclipses occur relatively frequently, it is not always possible to observe a blood Moon. In fact, the Red Moon is not a very common event . Two of these events occurred during 2014 and it was possible to observe it from North America, South America, Hawaii and New Zealand.
On July 27, 2018, it will be possible to observe what will be the longest total lunar eclipse expected for the 21st century and will coincide precisely with a full Moon period. The total phase is expected to last one hour and 43 minutes and the Moon can be seen with a red or reddish coloration. This event will be visible from Europe, Africa, Asia and in certain periods, from Australia. It is expected to start at 9:30 p.m. until 11:20 p.m., although the total eclipse will be reached around 10:20 p.m. There will also be a before and after period in which the Moon will be located in the lightest part of the Earth’s shadow, the so-called penumbra. Adding this period, the eclipse will last approximately 4 hours. In Spain, this eclipse will be visible from practically all cities.