Astronomy Answers
From the Astronomical Dictionary

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the month ⇒

A month is a period of time that is (historically) tied to the motion of the Moon around the Earth. Quite a few different kinds of months are in use:

• the calendar month: a period of between 28 and 31 days that divides calendars. In some calendars, such as the Gregorian calendar that is used in the West, the months are no longer directly tied to the phases of the Moon. In other calendars, such as the Jewish and Islamic calendars, each month starts with a specific phase of the Moon.
• the sidereal month: the period after which the Moon has returned to the same position (longitude) between the stars.
• the synodical month: the period after which the Moon attains the same phase again.
• the tropical month: the period after which the Moon returns to the same position relative to the vernal equinox.
• the anomalistic month: the period after which the Moon returns to the same position relative to its perigee.
• the draconic month: the period after which the Moon returns to the same node of its orbit.

De next table shows the average length of the astronomically determined months at 1 January 2000, and by how much this length changes in a century (derived from Meeus: Astronomical Algorithms, ISBN 0943396352). For example, on 1 January 2000 the average length of the synodical month was equal to 29 days, 12 hours, 44 minutes and 2.877 seconds, and this length increases at 0.019 seconds per century.

Table 4: Month Lengths

Month	days	d	h	m	s	s/century
sidereal	27.32166155	27	07	43	11.558	+0.019
synodical	29.53058885	29	12	44	02.877	+0.019
tropical	27.32158224	27	07	43	04.706	+0.013
anomalistic	27.55454988	27	13	18	33.109	−0.090
draconic	27.21222082	27	05	05	35.879	+0.033