Sidereal vs. Synodic Month
- The Moon completes an orbit around the Earth once every 27.3 days.
- This time period is called the ``Sidereal'' lunar month. It is the "true" orbital period of the Moon about the Earth.
- The time interval between New Moons is 29.5 days???
- A New Moon occurs when the Moon lies between the Sun and the Earth.
- As a lunar month progresses the Earth moves along its own orbit
around the Sun.
- After 27.3 days (i.e. a sidereal month) the Earth has moved to
a new position in its orbit. The Moon must travel along its orbit for an
additional 2.2 days in order to re-align with the
Sun and thus become ``New'' again.
- This time interval between New Moons is called the ``Synodic''
Updated September 24, 2003