What is the moon's orbital period - Sun-Earth Days
What is the moon's orbital period?
The moon moves in an orbit around the Earth about once every month. But the moon actually has two different orbital periods depending on the location of the observer. So, let's first understand what we mean when we say: "The moon goes around the Earth".
SIDEREAL PERIOD
The sidereal period or month is the time it takes the moon to move one full circle (actually an ellipse) around the Earth (360 degrees). It is the time it takes the moon to return to the same position against the background of stars; as if viewed by an observer outside our solar system. A sidereal month is equal to 27.3 days.
SYNODIC PERIOD
The moon's synodic (pronounced "si nod ik") period is the time it takes it to go from full moon to full moon. This is longer than a synodic month because the Earth has advanced a little ( 360 degrees / 365.25 days per year = 0.9856 degrees per day) in its orbit around the sun. To go from full moon to full moon, the sun, Earth, and moon must lie in a straight line and return to a straight line, so the moon must go a bit more than 360 degrees to catch up with the Earth. A synodic month is 29.5 days.
You can calculate both the sidereal orbital period yourself !
You can calculate the orbital period of any object orbiting the Earth including the moon from the following equation which is simply Kepler's third law of motion (remember? P^2 ~ a^3) expressed by Issac Newton using his gravitational constant, u:
P = 2pi * sqrt (a^3 / u)
Where
P = orbital period ( in seconds)
pi = 3.14159....
a = semi major axis of the orbit (=384,400 km)
u = gravitational constant ( = 398,601 km^3 / sec^2 for Earth)
Now, plug in the above values to get the sidereal orbital period, P. Remember, the answer you get will be in seconds so make sure to convert to days.
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