The orbits

An orbit, the path of one celestial body, is formed while it revolves around anothersuch body.est une courbe décrite par un corps céleste.
One can observe a solar eclipse only when the Moon is located exactly between the Sun and the Earth, which is only possible during a New Moon when the Moon turns its dark side towards Earth. It is at this point that the three bodies are aligned in the Sun-Moon-Earth order.
Nevertheless, a solar eclipse does not occur every month. This is because the Moon is not always directly aligned with the Sun and the Moon during each New Moon :
the Earth moves around the Sun while following an elliptic almost circular orbit situated in a plane called the ecliptic, which is the orbit of the Earth around the Sun.
The Moon turns around the Earth while following an orbit which is not the same as the ecliptic. Its plane is slanted at a 5 degree angle in relation to the ecliptic. The Moon is therefor sometimes below and sometimes above the ecliptic.
During each revolution the Moon encounters the ecliptic in two spots called : knots.
One refers to ascending knots when the Moon goes from South to North of the ecliptic and of descending knots in the opposite case.
It is only when the Moon is situated close to one of these knots, at the new Moon, that its shadow can reach our planet and that it becomes possible to observe an eclipse at a given spot from the Earth. This explains a solar eclipse occurs on average every six months.

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Last modifications: 3 July'99