Ephemeris

source: Le Figaro, Sciences & Environnement

author: Tristan Vey

translation: doxa-louise

Why does the Moon turn red instead of disappearing

during an eclipse?

When passing into the the cone of shadow from the Earth, as will be the case Monday morning, the Moon looses some of its shine, but does not disappear. It does, however, take on a characteristic red hue responsible for the name ‘blood moon’.

During a total lunar eclipse, our satellite goes into the shadow cone made by the Earth to the point of being totally immersed. As a logical consequence, one might assume that our satellite should disappear. Yet this is not what happens. On the contrary, it takes on a lovely orange tint.

As an eclipse begins, the Moon does give the impression of being ‘eaten up’ by the invading shadow. But if one looks carefully, our satellite continues to reflect a bit of light from its dark side. This is not overwhelming so long as there is still a crescent exposed to the direct light of the Sun, because this aspect is dominant. But one is forced to admit that having been totally eclipsed, the Moon has not disappeared. It is certainly less bright, 20 000 to 100 000 times less so than the standard full moon, but one can still make it out perfectly. Its more or less dark color can range from greyish-maroon to bright orange, including intermediate colors from dark red to brick red.

The French astronomer André Danjon, who died in 1967, had devised an empirical scale going from 0 (quasi invisible Moon) to 4 (copper colored eclipse with a very shiny border) to qualify this tint and shine ( the two are linked).

How to explain this phenomenon? It is linked to a number of fundamental properties of light. As a ray of light changes environment, it becomes more or less somewhat deviated. This is called refraction. One can observe this easily enough by plunging a stalk in the bath water. The latter gives the impression of being bent from where it enters the water.

When the sun’s rays going to the sides of the Earth ( at the level of the limit between night and day) penetrate the atmosphere, they are thus slightly deviated. It is not by much but enough to go around the Earth and thus create a light beam that hits the Moon. And this light will be red because it goes through a very thick layer of atmosphere. And the atmosphere will absorb blue light and diffuse it ( which is the reason why the sky is blue). When the rays of the Sun go through a thick layer of air, the rays that reach us are left bereft of their colder colors. this is the reason why the Sun goes red when it goes down! The adjacent drawing summarizes well enough the situation.

Blood Moon

It is very difficult to predict exactly the color an eclipsed Moon will take on. This

depends on different parameters of the atmosphere ( temperature, level of humidity, composition, etc). ‘Under certain conditions, after a large volcanic eruption for example, there can be particles in the Upper Atmosphere that will block all light’, points out Pascal Descamps, astronomer at the IMCCE (Institut de mécanique céleste et de calcul des éphémérides). In that extreme case, the Moon could in fact disappear in the shadow of the Earth.

http://www.lefigaro.fr/sciences/2019/01/18/01008-20190118ARTFIG00128-pourquoi-la-lune-devient-rouge-mais-ne-disparait-pas-lors-d-une-eclipse.php