An eclipse occurs when a celestial body is put between two others. For example, Jupiter has 17 satellites, each time one of them passes between the Sun and the planet, an eclipse occurs.
For its part, in the small system formed by Jupiter and its satellites eclipses occur every day, because Jupiter also passes in front of its moons and prevents us from seeing them, that is, eclipses them.
Eclipses are due to the opaque bodies of the Solar System casting a shadow in space. The Earth also projects a shadow, and when the Moon passes through it, an eclipse of the Moon occurs.
The apparent sizes of the Moon and the Sun are the same; that is to say, that these stars from a distance look more or less the same width as your thumb can see if you look at it with the extended arm. Because of the similarity of its apparent size, sometimes the Moon passes right in front of the Sun and covers it, producing an eclipse of the Sun.
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