Month
From ArticleWorld
The word month is derived from the same root as the word ‘’moon’’. It is the approximate amount of time it takes for the moon to go through a complete set of phases, from new to full and back to new again. The moon goes through these phases as it travels around the earth. The estimated time for this transition is 29.5306 days.
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Scientific lunar months
Synodic
The ‘’synodic month’’ is the observed, roughly 29.5306-day cycle of the moon as seen from Earth. This number is an averaged quantity, because the synodic month varies from cycle to cycle. This month is also different from the actual cycle or sidereal month, as seen from outside of the earth. The synodic month varies because as the earth moves around the sun--changing the relative angle of sunlight hitting the moon--the moon must rotate a little past its actual starting point in order to appear as it did at its starting point.
Sidereal
The ‘’sidereal month’’ is the objective cycle time of the moon around the earth as measured against the stars. This time is measured to be approximately 27.322 days. Ancient astronomers were able to observe this orbital period by comparing the motion of the moon to the unmoving stars in the background.
Tropical
The ‘’tropical month’’ is about 6.8 seconds shorter than the sidereal month, and is based on the lunar equinox as it moves along the earth’s ecliptic.
Draconic
The ‘’draconic month’’ is approximately 2 hours 37 minutes and 35.7 seconds shorter than the sidereal month. It is the time it takes the moon to orbit from the point where the plane of lunar orbit meets the earth’s ecliptic plane and back again. Because these two planar intersections rotate around the earth, the draconic month is considerably shorter than the sidereal month. When the moon is at either of these intersections, and also directly between the sun and the earth, a solar eclipse will occur.
Anomalistic
The ‘’anomalistic month’’ is calculated by taking into account the elliptical nature of the moon’s orbital path. The ends of this ellipse move around the earth in a regular, approximately 9-year cycle. The point on the ellipse closest to the earth is called the perigee, and the point farthest away is the apogee. Because these two points rotate around the earth in the direction of lunar rotation, it takes 5 hours, 35 minutes and 21.7 seconds longer than the sidereal month for the moon to move in a complete elliptical orbit from perigee back to perigee or from apogee to apogee, as these points move away from the moon during every orbit.
Calendar months
The Gregorian calendar, the most widely used in the world, has 12 months in a solar year. They are:
- January: 31 days
- February: 28 days (29 during leap years)
- March: 31 days
- April: 30 days
- May: 31 days
- June: 30 days
- July: 31 days
- August: 31 days
- September: 30 days
- October: 31 days
- November: 30 days
- December: 31 days