Expert: Tom Whiting - 6/15/2004

Question
Hi

How long does it take the moon to travel around the earth and how do they calculate this and when was this calculation made

Thanks


Answer
Hi Ilan,
It does not have to be calculated...it can be done by
simple observation.  In fact, the moon's motion around
the Earth (which in itself is a misnomer because the
Earth-Moon system both revolve around the bari-center...
the exact center of gravity between the two bodies)
is so involved and  complicated, the easiest way to do it
is by simple observation.

One can time the phases from exact full moon to full moon,
and you'll come up with what is called the average synodic
month of 29.5306 days, but due to the elliptical orbit,
can vary from 29.25 and 29.75 days.

OR, you may be talking about the length of time relative
to the stars, called Sidereal month, of which the *mean*
value, relative to the starry background is 27.3216 days,
again can be done by observation, and of course this varies
by the same reasoning as the synodic period (month).

Then there is the anomalistic month, which is measured
from perigee to perigee, and this *beats* with a mean period
of 27.55 days, slightly out of phase with the other two
above.

Then there is the Draconic month which is time intervals
between the moon's ascending node on the ecliptic, and its
mean value is 27.122 days.

And we still haven't even covered minor movements like
libration and perterbations which allows the moon to
tilt its N or S pole toward us or away from us, or where
the rotational speed gets slightly ahead of, or behind,
the revolution phase, thereby allowing us to see up to
59% of the lunar surface over a period of time.

Then there is the 'up and down' motion of the moon relative
to the ecliptic, where the moon returns to the almost exact
same spot 6585.32 days...also called the Saros cycle,
and explains why eclipses repeat at regular intervals
at 18 yr 10 days and 8 hours later...the 8 hours being
very important as the Earth rotates 120 degrees in that
time period, so the same eclipse does not occur at the
same Earthly spot, but is displaced 120 degrees westward,
some 18 years and 10 days (or 11, depending on number of leap years)later.

When?
All these observations were known by the early Greeks,
Babylonians, Chinese, Romans, and Egyptians...as they had
a lot of time on their hands.  The Saros cycle of eclipses was even known back then.

But of course, if you just want a "simple" but less accurate
way to do it, the moon moves about its own diameter
EASTWARD every hour, or 1/2 degree.  So the moon moves
about 12 degrees per 24 hour day, and if you multiply
12 x 29.53 days, you get very close to 360 degree complete
circle around the Earth. (Approximately)!

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA