Pyramid Science

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Monday, October 12, 2009

Sidereal And Synodic Time



Caution: needs review (23.01.2017)

Orbital Period

There are two terms that should be considered when describing lunar phases: sidereal and synodic. The former is the time to complete an absolute rotation about Earth [no, orbits the Sun close to (and slightly faster) than Earth - DA]. Moon executes a single rotation about its own axis. The latter takes into account the rotation of the Earth about the Sun.
   In the apparent month (lunation - DA) it takes to move from one New Moon to the next New Moon or one Full Moon to the next Full Moon (phase to next same phase), the Earth has moved through 360/12 = 30 degrees about the Sun. To reach an equivalent phase, the synodic time is therefore approximated by:

390/360*27.32 = 29.59 days 

or

27.32 + (30/360*27.32) = [27.32 + 2.27] = 29.59 days

Orbits are not circular, but for the sake of argument this simplifies the concept.

Nevertheless, keep in mind that the complexity of Moon dynamics is greater than the simplistic view usually described in books.

  • This argument is not strictly correct. In a simplistic sense it serves a purpose and a more refined and sophisticated treatment is being prepared and is part of a much more extensive work

LB (13.10.2011)

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