<P> An anomalistic month is longer than a sidereal month because the perigee moves in the same direction as the Moon is orbiting the Earth, one revolution in nine years . Therefore, the Moon takes a little longer to return to perigee than to return to the same star . </P> <P> Draconic months are sometimes known as' draconitic' months or nodical months . The name "draconic" refers to a mythical dragon, said to live in the nodes and eat the sun or moon during an eclipse . A solar or lunar eclipse is possible only when the moon is at or near the point where its orbit crosses the ecliptic plane i.e. it is at or near one of the nodes . </P> <P> The orbit of the moon lies in a plane that is tilted with respect to the plane of the ecliptic at an inclination of about 5.3 ° . The line of intersection of these planes passes through the two points where the moon's orbit crosses the plane of the ecliptic: the ascending node, when the moon's path crosses the ecliptic as the moon moves into the northern celestial hemisphere and descending node when the moon's path crosses the ecliptic as the moon moves into the southern celestial hemisphere . The draconic or nodical month is the average interval between two successive transits of the moon through the same node . Because of the torque exerted by the sun's gravity on the angular momentum of the Earth - Moon couple, the plane of the moon's orbit gradually rotates westward, which means the nodes gradually rotate around the earth . As a result, the time it takes the moon to return to the same node is shorter than a sidereal month . It lasts 7001272122200000000 ♠ 27.212 220 days (27 d 5 h 5 min 35.8 s). The plane of the moon's orbit precesses 360 ° in about 6,798 days (18.6 years). </P> <P> A draconic month is shorter than a sidereal month because the nodes move in the opposite direction to that in which the Moon is orbiting the Earth, one revolution in 18.6 years . Therefore, the Moon returns to the same node slightly earlier than it returns to the same star . </P>

Do the lengths of the synodic month and the sidereal month vary from one lunar orbit to the next