How the ancient Egyptians determined that the day would have 24 hours – 07/23/2023 – Science

Humanity’s relationship with the measurement of time began before the appearance of the first written word. That is why it is a challenge today to investigate the origin of many units of time measurement.

However, some units of time derived from astronomical phenomena are quite easy to explain – and have probably been independently observed by many different cultures around the world.

For example, measuring how long a day or a year lasts is based on the apparent motions of the Sun relative to the Earth, while measuring months is derived from the phases of the Moon.

However, there are some measurements of time that are not clearly related to any astronomical phenomena. Two examples are week and time.

One of the oldest written traditions, Egyptian hieroglyphic texts, offer us a new perspective on the origin of the hour. It would have originated in the region spanning North Africa and the Middle East – and was adopted in Europe before spreading across the world in the modern era.

Time in Ancient Egypt

The Pyramid Texts, written before 2400 BC, are the earliest writings of Ancient Egypt. They include the word wnwt (pronounced “wenut”) – and a star hieroglyph associated with it. It follows, therefore, that wnwt refers to the night.

To understand why the word wnwt is translated as “hour”, it is necessary to travel in time to the city of Assiute in 2000 a. C. There, the inside of rectangular wooden coffin lids were sometimes decorated with an astronomical tablet.

This tablet contained columns representing periods of 10 days of the year. The Egyptian civil calendar had 12 months, each with three 10-day “weeks” followed by five days of festivals.

In each column, 12 star names are listed, forming 12 rows. The entire tablet represents changes in the starry sky over the course of an entire year, similar to a modern astronomical map.

These 12 stars are the first systematic division of the night into 12 temporal areas, each ruled by a star. However, the word wnwt it is never associated with those astronomical coffin tables.

It was only in the year 1210 BC, in the New Kingdom – the period of Ancient Egypt between the 16th and 11th centuries BC – that the relationship between the number of lines and the word wnwt became explicit.

astronomical instructions

One temple, the Osireion, at Abydos contains a wealth of astronomical information, including instructions on how to make a sundial and a text describing the movements of the stars. It also has a coffin-like star board in which all 12 rows are uniquely labeled with the word wnwt.

In the New Kingdom, there were 12 wnwt nocturnal and also 12 wnwt diurnal, both were clearly measures of time. The idea of ​​the hour is almost in its modern form, if not for two things.

First, although there were 12 hours of day and 12 hours of night, they were always expressed separately, never together as a 24-hour day.

The day was measured using shadows cast by the Sun, while the hours of night were measured primarily by the stars. This could only be done while the Sun and stars were visible, respectively, and there were two periods around sunrise and sunset that did not contain hours.

Secondly, the wnwt of the New Kingdom and our modern time differ in duration.

Sundials and waterdials show very clearly that the duration of wnwt varied throughout the year – with long night hours around the winter solstice, and long daytime hours around the summer solstice.

To answer the question of where the number 12 or 24 comes from, we need to figure out why 12 stars were chosen over a 10-day period.

Undoubtedly, this choice is the true origin of the hour. Was 12 just a convenient number? Perhaps, but the origin of the coffin astronomical tables suggests another possibility.

referential stars

The ancient Egyptians chose to use the bright star Sirius as a model and selected other stars based on their behavioral similarity to Sirius.

The key point seems to be that the stars they used as a reference for timekeeping disappeared for 70 days of the year, just like Sirius, although the other stars weren’t as bright.

According to the Osireion stellar text, every 10 days a Sirius-like star disappears and another reappears throughout the year.

Depending on the time of year, between 10 and 14 of these stars are visible each night. Recording them at 10-day intervals throughout the year results in a table much like the coffin star table.

Around the year 2000 BC, the representation became more schematic than accurate (to our understanding) – and a table with 12 lines appeared, giving rise to the tablets on coffins that we can see in museums in Egypt and elsewhere.

Therefore, it is possible that the choice of 12 as the number of hours in the night – and, finally, 24 as the total hours from noon to the next noon – is related to the choice of a 10-day week.

And so, modern time has its origins in a confluence of decisions that took place more than 4,000 years ago.

*Robert Cockcroft is associate professor of physics and astronomy; and Sarah Symons is Professor of Interdisciplinary Science, both at McMaster University in Canada.

This article was originally published on the academic news site The Conversation and republished here under a Creative Commons license. Read the original version here (in English).