How was it determined that a day is 24 hours long?

The calculation of the stars on the sky, based on which the ancient Egyptians determined that there would be 24 hours in a day.

Humanity's relationship with the measurement of time began even before the first written word. This is the reason why it is difficult for us to actually investigate the many units of measurement of time.
Since some of the units derived from astronomical phenomena seem fairly easy to define, it can be assumed that they have been used independently by many different cultures around the world.
It is a matter of the apparent motion of the Sun with respect to the Earth to measure the duration of a day or a year.

Where as months are measured, it is done according to the phases of the moon.

However, there are some measurements of time that do not exactly correspond to an astronomical phenomenon, and two examples are the week and the hour.

Egyptian hieroglyphs are one of the oldest written traditions. This gives us new information about the origins of the clock. It appears to have originated in North Africa and the Middle East region and was adopted in Europe before spreading throughout the world in modern times.

Time in Ancient Egypt

The inscriptions in the Pyramids of Egypt, written before 2400 BC, are the earliest known writings of ancient Egypt.
Among them is the word 'wnwt', which roughly reads 'wenut', and a hieroglyph associated with a star.
It seems that 'wnwt' is related to night.
To understand why the word 'wnwt' was translated 'hour', you have to travel to the city of Asyut.
The inside of the rectangular wooden lids of 2,000 BC coffins were sometimes decorated with astronomical tables or star directions.
This table had columns representing 10-day periods of the year. The Egyptian civil calendar consisted of 12 months, each consisting of three 10-day 'weeks', followed by five-day festivals.
Each column contains the names of 12 stars, making 12 rows. The entire table represents the changes in the sky throughout the year, like a modern star map.
Those 12 stars are the first systematic division of the night into 12 temporal regions, each ruled by a star. However, the word wnwt is never attached to these coffin star tablets.
The ancient Osirion Temple in Abydos, Egypt is a treasure trove of astronomical information
Astronomical Instructions
A temple called the Osirian at Abydos contains a great deal of astronomical information, including instructions for making a sundial and a text describing the movements of the stars. It also has a coffin-like star table with the word wnwt uniquely inscribed on all 12 rows.
The New Kingdom had 12 wnwt of night and 12 wnwt of day, both clearly measuring time. In them the concept of time is almost in its modern form, were it not for these two things.
Although day is 12 hours long and night is 12 hours long, they are always referred to separately, never written or called a 24-hour day.
The day was measured by and using the shadows cast by the sun, while the night time was measured primarily by the stars.
This can only be done when the sun and stars are in alignment, and there are two periods around sunrise and sunset with no hours.
Second, the New Kingdom's wnwt and our modern times differ in length.
The sundial and water dial show very clearly that the length of wnwt varies throughout the year. Long nights in cold weather and long days in summer.
To answer the question of where the number 12 came from, we need to know why the 12 stars were chosen for each 10-day period.
Undoubtedly, this choice is the real beginning of this time. Was 12 just a convenient number to choose? Perhaps, but there is another possibility.

The stars telling the time

The ancient Egyptians used the bright star 'Sirius' as a model and then selected other stars based on their similarity to Sirius.
The key point appears to be that the stars they used as timers disappeared for 70 days a year, just like Sirius, although the other stars were not as bright.
According to a text on the star Osirion, every 10 days a Sirius-like star disappears and another reappears, and this happens for a whole year.
Depending on the time of year, 10 to 14 of these stars are visible each night. Recording them at 10-day intervals throughout the year results in a coffin star table.
Around 2000 BC they became more common and a 12-row table was created, giving rise to the coffin table, which we can see in museums in Egypt and elsewhere.
Therefore, it is possible that the choice of 12 as the number of night hours and finally the choice of 24 as the total number of hours from noon to the next afternoon is connected to the choice of a 10-day week.
Therefore, our modern times began with a confluence of decisions that were taken more than 4000 years ago.
*Robert Cockcroft is Associate Professor of Physics and Astronomy and Sarah Simons is Professor of Interdisciplinary Science, both at McMaster University in Canada.
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