Chantley Srey, of King's High School, asks :-

Why do we have days?

Duncan Steel, a PhD graduate in physics from the University of Canterbury and the author of a book about the history and astronomy of the calendar, entitled Marking Time (Wiley, New York), responded.

Many people think that a day is how long it takes the Earth to spin on its axis. That is incorrect. We must make a little more than a complete rotation for the sun to rise again, because each day Earth moves along its orbit a bit, shifting the apparent direction of the sun. A year is close to 365.25 days long; in that time the Earth spins 366.25 times (one extra because of our orbit around the sun).

By convention (defined by an international conference in Washington in 1884) the day used in most countries starts and ends at midnight. Not all cultures and religions use that system. The Hebrew day begins at dusk, so that the Jewish Sabbath starts at about 6 pm on a Friday and ends at that time on Saturday. Similarly the Islamic month begins when the new moon is sighted soon after sunset, meaning that the Islamic day starts and ends when the sun goes down. Many cultures use sunrise as the start and end of the day.

Note that midday is the time when the sun crosses the meridian on only four days each year. The tilt of our spin axis and non-circularity of our orbit means that local mean time is up to 16 minutes away from the time indicated by the sun itself (as on a sundial), with an additional offset due to the time zone convention.

The seven-day week derives from a melding between Jewish religious beliefs (keep one day in seven sacrosanct) and the astrology of Mesopotamia (which recognised seven 'planets': Sun, Moon, Mercury, Mars, Venus, Jupiter, Saturn). This conjoining occurred during the Exile of the sixth century BC, when the Jews were moved to Babylonia from Judea. When they returned they took back with them the names of the days in the familiar order, an ordering based upon false beliefs about the relative planetary distances, and how each ruled specific hours of the day (hence the term 'horoscope').

After the Romans invaded Egypt between 63 and 30 BC the seven-day week idea spread through the eventual Empire. Before then the Romans had exclusively used an eight-day week. The seven-day week became a legality with an edict by Constantine the Great, a convert to Christianity, in AD 321.