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However, during every lunation, the Moon passes through the ecliptic twice, once on the way down (southwards) and once on the way up (northwards). These points where the Moon's orbits intersects the ecliptic are called the nodes. The node in which the Moon crosses the ecliptic from south to north is the ascending node, and opposite to this lies the node known as the descending node where the Moon crosses the ecliptic from the north to the south.
At least twice a year (sometimes three times a year) the nodes are closely aligned to the straight line between the Sun and the earth, forming a “eclipse window” period when an eclipse is possible. The alignment of the nodes denotes the ending and beginning of an eclipse season and is 173.3 days apart. Hence an eclipse year is 346.6 days long. If an eclipse year begins early in January, the next eclipse window will be in June/July and a further window will occur in late December. The years 2000 and 2001 had three eclipse windows.
An eclipse window lasts between 30 and 36 days centered on the nodal conjunction with the earth and Sun. The line between those two nodes rotates once very 18.6 years and this is important for another reason - the Saros cycle.
As mentioned above, the Earth and Moon cast their shadows into space. When a New Moon occurs in the vicinity of a node, it is possible for its shadow to fall onto the Earth, and we will see the Moon eclipsing or partially eclipsing the Sun. When a Full Moon occurs in the vicinity of a node it is possible for the Moon to pass through the shadow of the Earth, and we will see a part or the whole Moon eclipsed by the shadow of the Earth.
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