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Momentary occultation of a star
An eclipse occurs when a star is hidden from the eyes of the observer. This happens when one star positions itself between the observer and the star eclipsed (as when the Sun is hidden by the Moon) or when the eclipsed star moves into the shadow of another (as during eclipses of the Moon, when the Moon moves into the Earth's shadow and 'disappears' to the eyes of the observer on Earth).
Eclipses have long been mines of information for astronomers: until the advent of space telescopes, solar eclipses were dedicated to the observation of the Sun, as they were the only opportunity to study the solar corona, otherwise invisible because if the sun's intense radiation. Solar eclipses can only be observed from a very small area of the Earth, whereas eclipses of the Moon are visible from any point on the globe. Thus in a single lifetime, one has many more opportunities to see a lunar eclipse than an eclipse of the Sun, even though the latter are much more frequent. Eclipses (both solar and lunar) take place periodically, in the same order and during the same lunar months, according to an 18-year and 11-day cycle known as Saros.