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Nodal precession of the lunar nodes as the Earth revolves around the Sun causes an eclipse season approximately every six months. A lunar eclipse can occur only when the full Moon is near either lunar node (within 11° 38' ecliptic longitude), while a solar eclipse can occur only when the new Moon is near either lunar node (within 17° 25').
The Moon completes one orbit around the Earth in 27.32166 days. The two points at which the Moon crosses the ecliptic are known as its orbital nodes, shown as "N1" and "N2" (ascending node and descending node, respectively), and the line connecting them is known as the line of nodes.
In the case of objects outside the Solar System, the ascending node is the node where the orbiting secondary passes away from the observer, and the descending node is the node where it moves towards the observer. [5], p. 137. The position of the node may be used as one of a set of parameters, called orbital elements, which
An eclipse of the Moon or Sun can occur when the nodes align with the Sun, roughly every 173.3 days. Lunar orbit inclination also determines eclipses; shadows cross when nodes coincide with full and new moon when the Sun, Earth, and Moon align in three dimensions. In effect, this means that the "tropical year" on the Moon is only 347 days long.
A "deep eclipse" (or "deep occultation") is when a small astronomical object is behind a bigger one. [2] [3] The term eclipse is most often used to describe either a solar eclipse, when the Moon's shadow crosses the Earth's surface, or a lunar eclipse, when the Moon moves into the Earth's shadow. However, it can also refer to such events beyond ...
As a result of this nodal precession, the time for the Sun to return to the same lunar node, the eclipse year, is about 18.6377 days shorter than a sidereal year. The number of solar orbits (years) during one lunar nodal precession period equals the period of orbit (one year [ specify ] ) divided by this difference, minus one: 365.2422 / 18 ...
This causes an eclipse season approximately every six months, in which a solar eclipse can occur at the new moon phase and a lunar eclipse can occur at the full moon phase. Because the orbit of the Moon is inclined only about 5.145° to the ecliptic and the Sun is always very near the ecliptic, eclipses always occur on or near it.
So the Moon's orbit crosses the ecliptic at two points or nodes. If a New Moon takes place within about 17° of a node, then a solar eclipse will be visible from some location on Earth. [5] [6] [7] At an average angular velocity of 0.99° per day, the Sun takes 34.5 days to cross the 34° wide eclipse zone centered on each node.