About time: why do we have 12-day months, 7-day weeks, and 60-minute hours?

The process to get to how we measure time today has evolved with civilisations over thousands of years. It seems to come from human’s unique desire to manage time and categorise. Some things make sense, such as a day being one full rotation of the Earth and a year being the time taken for the Earth to rotate once around the Sun, which takes 365 and a quarter days. But why we have weeks and months seems a bit harder to comprehend, so here is a brief insight into how they came to be.

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Why are there 12 months in a year? Image: Unsplash/Eric Rothermel.

Why are there 12 months in a year?

The first Roman calendar came about in the 700s B.C., and actually had 10 months named Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. Martius after Mars, Aprilis after the Latin word aperit (‘to open’), Maius after Maia; daughter of Atlas, and Junius perhaps meaning ‘young’. The remaining latter months were derived from numbers. The months each had a purpose and there would be a change in attitude for each. The Romans based these months on the lunar calendar which they thought of as 29.5 days – so months were 29 or 30 days long. Since there were originally only 10 months, the remaining 60 or so days were unaccounted for until the later addition of Januarius and Februarius.

Large modifications to the calendar came with the reign of Julius Caesar in 46 B.C, who renamed the sixth month Julius and the fifth after Augustus Caesar. He also modified months to be either 30 or 31 days – making sure Julius and Augustus were 31 days – at the expense of Februarius. It is thought that February was made shorter as it was a month of honouring the dead; ‘februare’ literally means ‘to purify’.

Why are there 7 days in a week?

Not every civilisation had a seven day week originally. Egyptians had a 10 day week and the Romans had eight. It was the Babylonians in the 600s and 500s B.C. who chose seven due to the seven celestial bodies that they could observe (the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn). The prominence of the seven day week spread across the globe and was adopted by the Romans in A.D. 321. Some days of the week retain their names from Roman gods, whereas others have changed to Germanic Gods such as Tiu, Woden, Thor, and Fria. 

In Christianity, Sunday as a day of rest has long been observed but the concept of a weekend, which also includes Saturday, is relatively recent. Weekends only became official in the UK in the 1900s, and was mostly centred around efficiency of the workforce.

What about 60 seconds, 60 minutes, and 24 hours?

Grouping into the base of 60 is called the sexagesimal system and is still used today for time and angles. It is thought 60 was used rather than 10, for example, simply because of how many numbers are factors of 60. This was especially useful in interpreting degrees in a circle as well as sundials, which is how time was first measured. Groupings of 60 made managing points in time or on a circle much easier and allowed humans to become more organised.

With regards to hours in a day, this likely came around 1000 B.C., much before seconds and minutes. Light and dark periods of time were considered two separate entities as it wasn’t possible to measure time at night with a sundial. This was, however, until people began observing which stars were visible at what points in the night. In total, 24 stars were used and divided into two groups of 12 based on equinox days. This is how the concept of 24 hours in a day began.

An explanation of time. Source: Science Plus.

Where next?

On a very long-term scale, there is constant astrological change that will affect our calendar system. Firstly, that the Moon is gradually pulling away from the Earth which means that the Earth’s rotation is slowing over time. Days will therefore get longer, albeit at a very gradual rate of 1/75,000th of a second a year. Furthermore, scientists have found that the Earth is rotating around the Sun faster than recent years making the average day this year 0.05 milliseconds shorter than usual.

In terms of things we can control, there has been increased support for a four day working week in the UK. This is already being implemented in several countries, such as the Netherlands, and is likely to increase in popularity. It is thought that a reduced week is actually more efficient as it reduces burnout, reduces sick-days, and increases happiness. 

By Lara Shoesmith

Header image from Unsplash/Lukas Blazek