Calendopaedia - Months, Weeks, and Days


The word month is derived from the Old English word for moon. A month was originally the time between two new moons. Today astronomers refer to this period of time as a lunar month. Its average length is 29 days, 12 hours, 44 minutes, and 2.8 seconds, or 29.530589 days. The moon travels around the Earth in 27 days, 7 hours, 43 minutes, and 11.5 seconds. This is the sidereal month. The length is different to the lunar month because the earth is moving along its orbit around the sun. Calendar months usually differ in length, and all except February are longer than 29 days in order to accommodate the solar year, which is almost 11 days longer than a lunar year.

The names for the months in the present Gregorian calendar are taken from the ancient Roman months of the Julian calendar. January is derived from Janus, a household god of beginnings. He was often depicted facing in two directions. February was the time of a feast of purification called Februa. March was named after Mars, the god of war. April is of uncertain origin. It may be named after the Greek goddess Aphrodite. May is probably derived from the goddess Maia. June was named after the goddess Juno. July and August were named, respectively, after Julius Caesar and his successor, Augustus. The last four months got their names from their original numerical placement in the year. Septem, for instance, is Latin for "seven." See the Roman Calendar for details.


The origin of the seven-day week is not clear but it has no astronomical basis. It is likely that the luner cycle was the first recognised time unit greater than the day. This is because it is not too long to observe and it is easy to identify the cycle start with the first appearance of the new moon. The period thus measured is approximatley twenty nine and a half days (see above). This was probably devided into four (halved and halved again) to make a more useful measure. This would produce periods of seven and eight days, often three periods of seven and one of eight or nine waiting for the next new moon. It could have been regularised to seven days for consistancy or because The Israelites were told to do so by God. God said to Moses in Exodus Ch 20 'Six days you shall labour and do all your work, but the seventh day is a Sabbath to the LORD your God. On it you shall not do any work'.

The Emperor Constantine I introduced it to the Roman Empire in the 4th century AD. The days were named after the then known seven planets: Saturn, Jupiter, Mars, the sun (not distinguished from a planet at the time), Venus, Mercury, and the moon (also considered a planet). The names of days in Latin countries still point to these origins, as do Sunday, Monday, and Saturday in English. Tuesday, Wednesday, Thursday, and Friday, however, are named after the Scandinavian gods Tiw, Woden, Thor, and Frigga. See the English Calendar for more details.

I am aware of three exceptions to the seven day week. They are:-


The Day, in chronology, is that period of time required for one rotation of an astronomical object, especially the earth, on its axis. The time for the stars to reappear in the same position as they were the previous day is known as a sidereal day. The time for the sun to reappear in the same position as it was on the previous day is known as a solar day. A solar day is about four minutes longer than a sidereal day. This is so because as the Earth orbits the sun, the sun appears to move slowly eastward against the fixed stars. Thus, for an observer on Earth, it takes slightly longer for the sun to return to its original position in the sky than it takes for the stars. The length of the earth's solar day, measured by reference to the sun's position, is averaged over a year, and the mean solar day thus obtained is used for all civil and many astronomical purposes. The civil day begins at midnight, local time. In common usage, day is the period of natural light between dawn and dusk. The period of daylight, nearly constant near the equator, varies with the latitude and the season, reaching a maximum of 24 hours in the polar zones in summer.

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