The length of a day on Earth has not remained constant throughout its history. In fact, it has gradually changed over billions of years due to various factors, primarily the interaction between the Earth and the Moon.
The primary mechanism behind the change in Earth's rotation is tidal friction. Tidal forces between the Earth and the Moon cause a transfer of angular momentum, resulting in a gradual slowing down of the Earth's rotation. This process is known as tidal braking.
When the Earth was forming about 4.5 billion years ago, its rotation was much faster than it is today. The Moon also formed during this period through a giant impact event, where a Mars-sized body collided with the young Earth. The gravitational interaction between the Earth and the Moon caused the transfer of angular momentum and gradually slowed down the Earth's rotation over time.
As the Earth's rotation slowed, the length of a day increased. However, it's important to note that this process has not been entirely constant. Various factors, such as changes in the distribution of Earth's mass, volcanic activity, and other geological processes, can influence the rotation rate and cause short-term variations.
Currently, the Earth's rotation is slowing down at a rate of approximately 1.7 milliseconds per century. This means that the length of a day is increasing by about 1.7 milliseconds every century.
It's worth mentioning that the effect of tidal braking is relatively slow compared to the timescale of human existence. Therefore, the changes in the length of a day over the course of a human lifetime are negligible. However, over geological timescales, these changes become significant.