The day lasts 24 hours because that's the time it takes for the Earth to complete one full rotation on its axis.
The Earth rotates on its axis around an imaginary line in about 24 hours, which is called Earth's rotation. This rotation defines what is known as the solar day, meaning the time it takes, as seen from Earth, for the Sun to return to the same position in the sky. It is mainly due to this rotation that we observe the regular day/night cycle: on one side of our planet, it is daytime, and on the other side, well, it is nighttime. Although it seems simple, the duration of a rotation is not exactly 24 hours, it actually lasts about 23 hours, 56 minutes, and 4 seconds (this is the sidereal day). The 24 hours we refer to in daily life comes from considering the position of the Sun relative to a specific location on Earth, and not just the rotation itself.
This idea of a day divided into 24 hours comes from the ancient Egyptians. They used a base of twelve, because they could easily count on the joints of their fingers (three joints per finger, four fingers, that makes twelve!). They split the day into two parts of twelve hours each: twelve hours for the night, twelve for the day. Then, the Babylonians added their input. They loved counting in base sixty: that's where we get the 60 minutes and 60 seconds in our modern hours. Later, the Greeks and then the Romans adopted this practical division, combining it all. And today, we still find ourselves living with these ancient Babylonian and Egyptian choices. Not bad for decisions that date back thousands of years, right?
The actual duration of a solar day (the time elapsed between two successive solar noons) is influenced by the orbital motion of the Earth around the Sun. The Earth does not only rotate on its axis: it also moves along its orbit during this time. Because of this, our planet has to make a small additional rotation each day to bring the Sun back to the same spot in the sky, which slightly lengthens the duration of the solar day compared to the sidereal day (which is based solely on the Earth's rotation). The speed of this orbit is not constant, as the Earth's orbit is elliptical, that is to say slightly oval, and not perfectly circular. As a result, at certain times of the year, the Earth moves faster along its orbit, and at other times more slowly, which creates slight but real variations in the exact duration of the solar day. These small differences accumulate and give rise to the equation of time, an annual correction between true solar time (indicated by a sundial) and the standard time displayed by our watch.
Even if we consider a day to be exactly 24 hours, our planet sometimes has its own way. Not completely regular, the Earth's rotation is influenced by several astronomical phenomena. Take the example of ocean tides caused by the gravitational pull of the Moon (and to a lesser extent, the Sun). These tides result in a very slight (but continuous) slowing of the Earth's rotation, gradually lengthening the day. Another factor is the internal movements of the Earth's mass, such as the shifting of magma deep beneath our feet (it moves down there!), which subtly alters the rotation speed. The result is that the exact length of a day regularly varies, sometimes necessitating the exceptional addition of a leap second to stay in sync with the official time based on atomic clocks.
The actual solar day (the exact time it takes for the Earth to face the sun again) is not exactly 24 hours. It varies slightly throughout the year, primarily due to the Earth's elliptical orbit around the Sun, which can show variations of several seconds or even minutes.
A leap second is sometimes added to our calendar, allowing us to compensate for the irregular variations in the Earth's rotation speed caused by various astronomical and geophysical factors.
The division of time into 24 hours comes from the ancient Egyptians, who already used a system of twelve units for the night, to which they added twelve units for the day. This system was later passed on to other ancient civilizations, such as the Babylonians.
The expression "to be in the moon" comes from the observation of lunar cycles, which often disrupted the perception of time among ancient peoples, who were less accustomed to the precisely measured rhythms we use today.
Yes, depending on the season and geographical location, the duration of the day (sunshine period) varies greatly. This is due to the tilt of the Earth's axis in relation to the Sun, which results in longer days in summer and shorter days in winter.
The sidereal day corresponds to the period during which the Earth makes a complete rotation on its axis relative to the fixed stars. However, since the Earth revolves around the Sun while rotating on its axis, it must turn a bit farther to bring the Sun back to the same position in the sky, which slightly extends the duration, thus defining the solar day.
No, in reality the time it takes for the Earth to make exactly one complete rotation on its axis is slightly less than 24 hours (about 23h56min4s). This is called a sidereal day. However, the complete rotation of the Sun in the sky (solar day) lasts on average exactly 24 hours due to the simultaneous movement of the Earth around the Sun.
Coordinated Universal Time (UTC) is used as an international reference for synchronizing global clocks, as it is based on the mean time at the Greenwich meridian. It thus ensures consistency and practical accuracy in international exchanges and global scientific calculations.
The choice of a sexagesimal base (base 60) for dividing time comes from ancient Mesopotamian civilizations, such as the Babylonians, who appreciated this very practical mathematical system for division. It was then widely adopted, as it allows for many whole subdivisions, thereby simplifying various astronomical and daily calculations.
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