Eight days a week

I find it useful to think back to what I was doing this day in previous weeks, until I began this process of remembering every day that passes.

A week is a useful period of time, but it is the one aspect of the calendar that is totally arbitrary.

A year is given by the time it takes the earth to orbit the sun - and before the earth's orbit was understood, it was given by the cycle of seasons that results.

A month is given by the time it takes the moon to complete the passage of its phases, linked to its orbit of the earth.

A day is given by the time it takes the earth to revolve once on its own axis.

All were observable by all cultures.

But a week is not set in the heavens, though it is set in the old testament of the Bible of Christians and Torah of Jews.

Other civilisations have used other lengths for the cycle between a day and a month, from four to ten days, apparently.

Seven days almost fits into a lunar month four times, but not precisely enough to be a realistic explanation of its origin.

It may have more to do with our capacity to remember sets of data, which has been said to have a natural limit of seven - though plus or minus two according to some research.

The Beatles sang of eight days a week as cramming in more love than a week normally allows, but there is no clear reason why the week is not eight days. It was in ancient Rome under the nundinal system, until Emperor Constantine adopted the biblical seven days.