February 09 2017
The Moon circles the Earth on the lunar orbit. The Earth circles the Sun on the Ecliptic orbit. During a complete revolution of the Moon around the Earth, our satellite goes through the plan of the ecliptic twice. So during half of this revolution, the Moon moves slightly above the Earth-Sun axis, and underneath it during the other half. These precise moments of passage through the plane of the ecliptic are called lunar nodes. More precisely descending node when the Moon crosses through to move under the Earth-Sun axis, ascending node when it gets over it.
The length of time between two identical lunar nodes, approximately a little over 27 days, is called the draconic cycle.