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I know that the Moon's orbital plane makes an angle of 5 degrees with the Earth's orbital plane or the ecliptic. Also, the Moon's orbit intersects the Earth's orbital plane at two nodes - ascending node and descending node. Hence, a lunar node is generally defined as either of the two points at which the orbit of the Moon intersects the ecliptic.

Doesn't this mean that the Moon's orbit intersects the Earth's orbit around the Sun at the orbital nodes? If that's true, why isn't a lunar node defined simply as the point of intersection of the Moon's orbit with the Earth's orbit? Why do we need to bring the 'ecliptic plane' into the definition of the lunar node?

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Doesn't this mean that the Moon's orbit intersects the Earth's orbit around the Sun at the orbital nodes?

Not in the general case. Consider an arrangement where the lunar nodes are oriented towards and away from the sun. The moon will be above (or below) the ecliptic when it crosses Earth's orbit ahead of the Earth, and below (or above) the ecliptic when it crosses Earth's orbit behind.

I believe, though, that the inclination plane of the moon remains (mostly) space-fixed as the Earth moves around the sun, so twice a year the lunar nodes will intersect Earth's orbital path.

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    $\begingroup$ The ecliptic is Earth's orbital plane. When the Moon passes through its ascending or descending node it's in the ecliptic, but (of course) that doesn't imply that its trajectory is intersecting Earth's. The Moon's orbital plane precesses quite quickly: the nodal precession period is ~18.6 years. Please see en.wikipedia.org/wiki/Lunar_precession which discusses the 3 types of lunar precession. $\endgroup$
    – PM 2Ring
    Aug 19 '21 at 20:34
  • $\begingroup$ Thanks! That was helpful. $\endgroup$ Aug 20 '21 at 4:48

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