why will the moon change color this monday?

why will the moon change color this monday?

If the Moon is adorned with exceptional colors during its total eclipses, it is because of our good old atmosphere.

On the night of Sunday May 15 to Monday May 16, lovers of space and grandiose shows of all kinds will be able to attend a visually sumptuous event recalled by Futura: a total lunar eclipse which will have the particularity of giving a magnificent reddish complexion to our neighbour. A good opportunity to look at the origin of this phenomenon sometimes called “Blood Moon”.

This is a relatively rare event that consists of a perfect alignment of the Sun, the Earth and the Moon in that order, so that the latter passes directly into the cone of shadow of the Earth. And it will be necessary to take advantage of it, since the French public had no longer been entitled to this show since January 21, 2019.

This rarity is due to the specificities of the tango of the Earth and its satellite. Indeed, the orbits of the Earth around the sun and of the Moon around the Earth are not not exactly on the same plane; the latter is inclined by 5° with respect to the first. This means that quite often when the three bodies might appear aligned, the Moon actually tends to pass. above or, conversely, below the famous shadow conethus depriving us of the spectacle.

A rare and spectacular coloring

On the other hand, on the night of Sunday to Monday, the Moon will pass this time in the right place. She will gratify us with her presence right in the middle of the Earth’s cone of shadow. And she will even release her evening dress for the occasion; she will put on a superb scarlet dress that would almost make her look like Mars.

This color is caused by a phenomenon called “Rayleigh scattering”, after the English physicist who discovered it. Even if the name means nothing to you, you are still familiar with the phenomenon even without having heard of it; it is because of this Rayleigh scattering that the sky is blueor that it is adorned with shimmering colors during a sunset.

The Rayleigh scattering which gives its color to the Moon during total eclipses is also at the origin of these superb light effects. © Boticario – Wikimedia Commons

When sunlight enters the atmosphere, it goes from an almost absolute vacuum to an environment littered with particles of gas, water and dust just waiting to deflect photons from their path. This results in a phenomenon of scattering of light. But this does not all affect the light spectrum in the same way.

A wavelength story

This difference in treatment is directly linked to the very nature of light. In essence, light is nothing but a cocktail of electromagnetic waves whose wavelength (very vulgarly, the distance between two “waves” of this wave) belongs to the so-called “visible” domain. , which ranges from violet (~380 nm) to red (700+ nm).

The shorter this wavelength, the faster the wave in question oscillates; and the faster it oscillates, the more likely it is to come into contact with a foreign body like a dust particle. It means that the smallest wavelengthswhich correspond to the pole purple/blue of the spectrum, will be broadcast more strongly.

The total lunar eclipse of January 21, 2019 photographed by J. Normand. © J. Normand via IMCCE

This scattering phenomenon means that the light rays are scattered in all directions; from the point of view of a terrestrial observer, they therefore go “lost” in space, far from the eyes of the public. In contrast, higher wavelengths located at the red end of the spectrum will have peasier to make their way through the atmosphere.

Upon entering and leaving the atmosphere, the reddish component of sunlight then undergoes a phenomenon of refraction more marked. This is another phenomenon that forces a wave to change its trajectory when it passes through a new medium with different physical properties, such as a laser beam that is passed through water. When the light leaves the atmosphere to rejoin the sidereal void, it therefore finds itself deflected towards the moonwhich gives it its beautiful color.

The latter is also not fixed. For the record, there is even a so-called Danjon scale, named after the French astronomer who defined it, to qualify the color of a total eclipse. It can go from dark brown (0) tocopper orange (4) passing through the brick red (3) and a whole bunch of more or less orange intermediates.

When to observe this eclipse?

But to be able to precisely determine the position of this weekend’s eclipse on the Danjon scale, it will be necessary to start by getting up at dawn; French people in mainland France will have to make some sacrifices in terms of their sleeping time.


As this infographic from the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE) shows, residents of France will witness the arrival of the Moon in the dark around 3:30 a.m. The total eclipse as well as the spectacular colors will not arrive until two hours later, around 5:30 a.m.. According to meteogram.fr, the Moon will then set around 6:15 a.m. The French in mainland France will therefore have the opportunity to observe it for about three quarters of an hour. West Indians and Guyanese, on the other hand, will be able to enjoy it much longer!

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