Eclipses, almost anyone could tell you, happen when the Moon, Earth and Sun align and our planet and its companion cast their shadows on each other:
in one case, the Moon obscures the Sun, and the eclipse is solar;
in the other, it is the Moon passing through the shadow of the Earth and then the eclipse is… lunar.
solar eclipses
Three types of solar eclipses can be observed:
They occur when the entire solar disk is obscured by the Moon. When this happens, the Moon is at the exact right distance to appear the same size as the Sun: it is then 400 times closer to the Earth than the Sun, but also 400 times smaller. The moment when the Moon perfectly obscures the Sun is called “totality” and can last up to seven minutes.
Minutes during which we can observe the atmosphere of our star: the chromosphere (lower atmosphere) and the solar corona (upper atmosphere, diffused over several million kilometres). This spectacle can be observed with the naked eye without danger, unlike the previous moments and succeeding totality, during which the sun’s rays (especially ultraviolet and infrared) are still as dangerous for the eyes, despite the drop in brightness of the Sun, and can cause irreversible eye damage. Be careful, therefore, to bring appropriate protective glasses.
Partial eclipses:
They occur when the Moon only partially obscures the Sun, passing a little too high or low in relation to it.
Annular eclipses:
They occur when the Moon is too far from Earth to completely obscure the Sun, leaving a ring of light visible around the Moon. They are slightly more numerous than total eclipses, because the Moon is often a little too far from the Earth.
Lunar eclipses
Much less known because less spectacular, lunar eclipses can be much longer than solar eclipses because the shadow of the Earth, in which the Moon enters, is much wider. Since the Moon is currently close to the most distant point of its orbit (known as the apogee), its speed of movement in its orbit is lower, which means that on July 27, it should remain in the terrestrial shadow for about a hundred three minutes according to the calculations of the astronomers, not very far from the theoretical maximum duration (107 minutes).
Just like its solar counterpart, the lunar eclipse can be observed in three configurations:
penumbral eclipses: this is the least visible eclipse, since the Moon does not enter the Earth’s shadow but its penumbra. Clearly, it is well hidden from the Sun but not in the shadow cone. The lunar surface then darkens slightly, but not clearly.
partial eclipses: just like the partial solar eclipse, here the body of the Moon is partially masked by the shadow of the Earth and the lunar disc is partially plunged into darkness
total eclipses: in this configuration, the Moon enters completely into the cone of shadow of the Earth and its visible surface darkens more or less. It often takes on an orange or even red hue, which is commonly called the “Blood Moon”. This tint, specific to total lunar eclipses, is due to the sunlight which passes through our atmosphere on each side of the Earth before being refracted and reaching the Moon. It is in a way the aggregate of the sunsets and sunrises which occur respectively in the west and in the east that we then find on our natural satellite.
Why so few eclipses?
Since our natural satellite completes its orbit around the Earth in 29.5 days, and during its journey it necessarily aligns with the Earth and the Sun, how come we only have two to five eclipses per year, not two per month?
This is explained by the inclination of the plane in which the Moon revolves around the Earth with respect to the Earth-Sun plane (the “plane of the ecliptic”). An inclination of 5.1°, precisely. This causes the Moon’s shadow to cast well every month, but very often above or below our planet.
The orbit being inclined, it crosses the Earth-Sun plane in two places, these are the lunar nodes. If the Moon, when it passes over one of these two nodes, is aligned with the Earth and the Sun, then an eclipse occurs. There are at least two solar eclipses per year, but some years have more (up to five! The next year with five eclipses being… the year 2206).
Total solar eclipses are set to disappear
At the rate of three centimeters per year, the Moon is gradually moving away from the Earth. There will come a time, some 650 million years from now, when it will have become too small to entirely hide the Sun King.