Michael Steward, of King's High School, asks :-
Why do we have months and who invented them?
Duncan Steel, a PhD graduate in physics from the University of Canterbury, author of the book Eclipse, (Hodder Headline), contains much material about month lengths and how they are defined, responded.
Our month lengths and names stem from the Roman era. Like many civilisations the Romans initially used months which followed the moon, hence the word `month'. The period between successive full moons averages to 29.5306 days (although with a possible range of eight hours about this value). Months based on the moon would therefore last for either 29 or 30 days, basically alternating but with a few more 30s than 29s to produce an average above 29.5. For example, some religious calendars use a system in which 19 years are split into 235 lunar months, seven of the years having 13 months and a dozen have twelve months each, the 235 being split as 110 of 29 days and 125 of 30 days. The date of Easter is based on such a 19-year cycle.
We do not know when the Romans abandoned the moon with regard to the month length, but it was before 400 BC. Thereafter the Roman calendar was very erratic, with a 13th month sometimes being inserted into February according to the whims of the senate. By 153 BC the twelve months as we use them were in their present order, although with different lengths. January and February were moved to the start of the year; thus the names of September through December have names indicating them to be the seventh through tenth months, which they are not.
Laxity in inserting the 13th months put the Roman calendar out of step with the seasons. In 46 BC Julius Caesar declared a new calendar, making that one year 455 days long so as to make up for the previous errors. This new (Julian) calendar was to have single days inserted every fourth year, rather than whole extra months.
An alteration of this scheme occurred in AD 1582 when Pope Gregory XIII introduced what is known as the Gregorian calendar. This has century years (those ending 00) not being leap years unless they are divisible by 400. Thus AD 2000 is a leap year, but 1900 was not, nor will 2100 be a leap year. The reason for this alteration was to regularise the date of Easter. Catholic nations adopted this calendar immediately, whilst Protestants tarried. Britain did not make the change until 1752. The calendar used in New Zealand derives from the British legal code, which was inherited by the new colony/dominion.
It was Julius Caesar who defined the month lengths which we now use. After his assassination in 44 BC, the senate re-named July for him. Later the following month was re-named August, for Augustus Caesar. Much later other emperors such as Claudius tried to name months for themselves, without long-lasting success.
Whilst the familiar months are recognised by most people, other cultures use quite different schemes. The Jewish festival of Passover is on the 14th day of the month of Nisan, on the Hebrew calendar. Similarly the traditional Chinese calendar has New Year starting on the second dark of moon after the December solstice.
Some people still use a 'lunar year', as in the Islamic calendar. One Islamic year contains twelve lunar months, and so lasts for only 354 or 355 days. Because of this that the Ramadan slips backwards by 11-day steps through our own calendar, shifting through all the seasons over 33 or 34 of our (solar) years.