Why 24 hrs in a day

Why do our clocks measure out time the way they do? We probably take it for granted, but there are lots of different ways to measure the passing of time: the one we use just won out over all the other ones.

Like the Gregorian calendar, which we discussed in this episode of DNews , the clock we use has gradually evolved over time. Since the metric system uses base of 10, it may seem like the obvious choice to base our time keeping on, but ancient cultures used all sorts of different bases. The duodecimal system, which has a base of 12, was popular probably because it takes 12 lunar cycles to make one trip around the sun.

Despite the fact that they're only actually equal on seasonal equinoxes, days and nights each got assigned 12 hours. The Ancient Babylonians take credit for the hour being made up of 60 minutes. For reasons that remain unclear, they used a base 60 system of counting.

So far, there have been 27 leap seconds, and they have all been positive. The table below shows the yearly average day lengths since Astronomers and timekeepers express mean solar time as Universal Time UT1 , a time standard based on the average speed of the Earth's rotation.

The speed of the Earth's rotation varies from day to day. One of the main factors are the celestial bodies surrounding us. For example, the Moon's gravitational pull causes tides and changes the Earth's shape, ultimately resulting in a lower rotational speed. The distance between Earth and Moon changes constantly , which makes for daily variations in the speed our planet rotates around its axis.

Super-accurate atomic clocks were first developed in the s and s. So measurements of the Earth's rotation using atomic clocks only go back as far as then. However, telescopic timings of stellar occultations by the Moon provide information about the Earth's rotation going back to the 17th century.

An occultation is when the Moon, as seen from the Earth, passes in front of a star. Our hour day comes from the ancient Egyptians who divided day-time into 10 hours they measured with devices such as shadow clocks, and added a twilight hour at the beginning and another one at the end of the day-time, says Lomb. The Egyptians had a system of 36 star groups called 'decans' — chosen so that on any night one decan rose 40 minutes after the previous one.

Amazingly, such tables have been found inside the lids of coffins, presumably so that the dead could also tell the time. In the Egyptian system, the length of the day-time and night-time hours were unequal and varied with the seasons.

The subdivision of hours and minutes into 60 comes from the ancient Babylonians who had a predilection for using numbers to the base Lomb says it's likely that the Babylonians were interested in because that was their estimate for the number of days in a year. Their adoption of a base 60 system was probably allowed them to make complex calculations using fractions. The ancient Chinese used a dual time system where they divided the day into 12 so-called, 'double hours', originally with the middle of the first double hour being at midnight.

They also had a separate system in which a day was divided into equal parts called 'ke', that are sometimes translated as 'mark' into English.

Because of this inconvenience, much later on, in the year of our era, the number of ke in a day was reduced to 96," says Lomb. While many cultures had their own calendars, there doesn't appear to be evidence for equivalent methods for keeping time. In , the Swiss watch company Swatch introduced the concept of a decimal Internet Time in which the day is divided into 'beats' so that each beat is equal to 1 minute The beats were denoted by the symbol, so that, for example, denotes a time period equal to six hours.

I think that I am safe in stating that there will be no change from the present system of time measurement in the foreseeable future.