Month

From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age.

Synodic months, based on the Moon's orbital period with respect to the Earth–Sun line, are still the basis of many calendars today and are used to divide the year.

Most of them (but not the distinction between sidereal and tropical months) were first recognized in Babylonian lunar astronomy.

[citation needed] The Sun moves eastward with respect to the stars (as does the Moon) and it takes about 2.2 days longer for the Moon to return to the same apparent position with respect to the Sun.

At the simplest level, most well-known lunar calendars are based on the initial approximation that 2 lunations last 59 solar days: a 30-day full month followed by a 29-day hollow month — but this is only roughly accurate and regularly needs intercalation (correction) by a leap day.

Additionally, the synodic month does not fit easily into the solar (or 'tropical') year, which makes accurate, rule-based lunisolar calendars that combine the two cycles complicated.

However, a Metonic calendar based year will drift against the seasons by about one day every 2 centuries.

Purely solar calendars often have months which no longer relate to the phase of the Moon, but are based only on the motion of the Sun relative to the equinoxes and solstices, or are purely conventional like in the widely used Gregorian calendar.

Therefore, the beginning and lengths of months defined by observation cannot be accurately predicted.

While some like orthodox Islam and the Jewish Karaites still rely on actual moon observations, reliance on astronomical calculations and tabular methods is increasingly common in practice.

The last three reformed Roman calendars are called the Julian, Augustan, and Gregorian; all had the same number of days in their months.

The Gregorian calendar, like the Roman calendars before it, has twelve months, whose Anglicized names are: The famous mnemonic Thirty days hath September is a common way of teaching the lengths of the months in the English-speaking world.

The knuckles of the four fingers of one's hand and the spaces between them can be used to remember the lengths of the months.

This physical mnemonic has been taught to primary school students for many decades, if not centuries.

The calends are always the first day of the month,[a] and before Julius Caesar's reform fell sixteen days (two Roman weeks) after the ides (except the ides of February and the intercalary month).

The Tongan calendar is based on the cycles of the Moon around the Earth in one year.

The Moon first appears in March,[clarification needed] they name this month Kahlek.

Each full moon Poya day marks the start of a Buddhist lunar month.

These months were attested by Bede in his works On Chronology and The Reckoning of Time written in the 8th century.

[14] His Old English month names are probably written as pronounced in Bede's native Northumbrian dialect.

This setup means the calendar could stay precisely aligned to its lunar phase indefinitely.

There were twelve months of 30 days each, grouped into three ten-day weeks called décades.

Additionally, Rhodes[19] also informs of not only the variability in the month names, but how in Eastern Ojibwe these names were originally applied to the lunar months the Ojibwe originally used, which was a lunisolar calendar, fixed by the date of Akiinaaniwan (typically December 27) that marks when sunrise is the latest in the Northern Hemisphere.