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The months The months CA month is a unit of time used with calendars which was ?rst used and invented in Sumer Mesopotamia as a natural period related to the motion of the moon In English month and moon are cognates In many languages the same word is used

The months CA month is a unit of time used with calendars which was ?rst used and invented in Sumer Mesopotamia as a natural period related to the motion of the moon In English month and moon are cognates In many languages the same word is used for month and moon Hebrew ? yerah yareah The traditional concept arose with the cycle of moon phases such months lunations are synodic months and last approximately days From excavated tally sticks researchers have deduced that people counted days in relation to the moon's phases as early as the paleolithic age Synodic months based on the moon's orbital period are still the basis of many calendars today and are used to divide the year Types of months Sidereal month The period of the Moon's orbit as de ?ned with respect to the celestial sphere of the ?xed stars nowadays the International Celestial Reference Frame ICRF is known as a sidereal month because it is the time it takes the Moon to return to a given position among the stars Latin sidera days d h min s Tropical month It is customary to specify positions of celestial bodies with respect to the vernal equinox Because of precession this point moves back slowly along the ecliptic Therefore it takes the Moon less time to return to an ecliptic longitude of zero than to the same point amidst the ?xed stars days d h min s This slightly shorter period is known as tropical month cf the analogous tropical year of the Sun Anomalistic month The Moon's orbit approximates an ellipse rather than a circle However the orientation as well as the shape of this orbit is not ?xed In particular the position of the extreme points the line of the apsides perigee and apogee makes a full circle lunar precession in about nine years It takes the Moon longer to return to the same apsis because it moved ahead during one revolution This longer period is called the anomalistic month and has an average length of days d h min s Draconic month Sometimes written 'draconitic' month and also called the nodical month The orbit of the moon lies in a plane that is tilted with respect to the plane of the ecliptic it has an inclination of about ?ve degrees The line of intersection of these planes de ?nes two points on the celestial sphere the ascending node when the moon's path crosses the ecliptic as the moon moves into the northern hemisphere and descending node when the moon's path crosses the ecliptic as the moon moves into the southern hemisphere The draconic or nodical month is the average interval between two successive transits of the moon through its ascending node Because of the sun's Cgravitational pull on the moon the moon's orbit gradually rotates westward on its axis 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