How many days in a luna month?

Question

How many days in a luna month?

Answer 1

There are more than one way to measure a lunar month.

The most common one is the time period that originally gave us the calendar month (still used in lunar calendars, such as the Chinese calendar).

It is the average time until the Moon returns to the same phase (New Moon to New Moon, or Full to Full).

29.530589 Earth-days (of 86,400 seconds each).

This is called the Synodic month (from an old word meaning "to announce" as the return of the New Moon "announced" the new month).

If you look at the Moon's orbit relative to fixed star, the orbital period is 27.321662 Earth-days.  This is the time it takes for the Moon to return in line with the same star.

It is called the Sidereal month (sider = star).

This is the period to use if you are doing celestial methanics (using Kepler's laws, for example).

There are other "months" but they are only used in very specialised calculations.

For example, the "Anomalistic" month (27.554550) is the time from perigee to perigee.  It is used when preparing the water level (tide height) portion of tide tables.

Answer 2

The average interval between full moons is 29.530589 days.

Answer 3

2-3

Answer 4

In Arabic lunar calender 5 months (2nd, 4th, 6th, 8th, 10th months) have 29 days and the rest 7 months - 30 days. The whole lunar year has 355 days.