iwc Da Vinci 3736 Mode D'emploi page 14

Lunar orbit of the Small Da Vinci
Expressed in simple terms, a month is the time taken
by the moon to revolve around the earth. Depending on
the reference point from which we observe the orbit,
the length of the months can vary. There are, in fact, no
heavenly bodies whose orbit has such a complicated
path as that of the moon. It is easy to suppose that a
month is up once the moon has completed 360 degrees
of its orbit around the earth. It is then back at the same
point before the firmament. This month (the siderial
month) lasts for precisely 27 days, 7 hours, 43 minutes
and 11.5 seconds.
In the meantime, however, the earth has continued
on its orbit around the sun. In order for it to assume the
same position in relation to the sun, the moon must
continue in its orbit for a distance beyond 360 degrees.
This month lasts longer, therefore, and we can only
use this month for calculating the length of the months
and also for our moon phase watch. It is known as
the synodic month. This period lasts for 29 days, 12 hours,
44 minutes and 2.9 seconds, or 29.530588 days
expressed as a decimal quantity. If we divide the length
of the year of 365.2422 days by this period, the result
can be rounded off to give 12.368 revolutions per year.
A consequence of the previously mentioned compli-
cated orbit of the moon is that the calculated (mean)
lengths of the months for the synodic month can differ
by up to 6 hours.
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