Expert: Tom Whiting - 1/4/2010

Question
Dear expert,

if one conceives an Earth-Moon system isolated from influence of the solar system, (the Sun consequently condisered as an another star), does moon's sideral period would become equal to its synodic period?


sincerely


Answer
No, the difference between the two periods is the 27.3 days (lunar sideral period) the Earth
revolves around the sun for those 27.3 days, so it takes another 2.2 days for the moon to
become aligned with the sun, relative to the Earth... ie..new moon phase to next new moon phase, or the synodic period.
The influence of the other planets has nothing to do with the Earth's annual revolution around
the sun. The Earth-moon system could care less about the rest of the Solar system (and them, us), the sun excepted. In fact, we are ALREADY pretty much an isolated system because the gravitational effects of the other planets is so minor, barely measureable by very sensitive equipment, let alone catastrophic.

So as long as a planet is orbiting a star... even Pluto's moon Charon, there would be a minor
difference between Charon's sideral and synodic period...not much, but a little, perhaps a
millisecond, because Pluto does advance around the sun slightly in their 3.6 day sideral orbit.

Theoretically, I guess,  IF the Earth-moon system were say, 100 ly out in space...with no sunlight to illuminate the lunar surface, then there would be no phases visible. So in that case, I guess the two periods would be equal, although technically, there would be no lunar phase illumination to even see a synodic period, to compare the two, right?  (relative to which star????)  Of course, there would not be any life on Earth in that situation anyway, at 3 degrees
above absolute zero, so it's pretty much an outlandish and meaningless hypothetical question.
Hope this helps,
Clear skies,
Tom Whiting
Erie, PA