Earth's Day Duration: A Look at the Past and Future

Once upon a time, the span of an Earth day was less than 20 hours. Between 2 billion to 600 million years ago, the day length on our planet stabilized around 19.5 hours. Researchers have now uncovered the reasons behind this.

In Earth's history, the duration of a single day was not always the equivalent of our current 24 hours. The day length has been gradually extending since the Moon's emergence. Presently, the day length is still expanding, but during a specific era — between 2 billion to 600 million years ago — the days were a mere 19.5 hours. As explained by a group of astrophysicists from the University of Toronto in the Science Advances journal, the balance of forces between the Moon and the Sun determined this.

The Moon's Influence on Earth's Day Length

The duration of a day is influenced by the speed of Earth's rotation on its axis. Before the Moon's formation, our planet's rotation lasted only a few hours. However, with the advent of the Moon roughly 4.5 billion years ago, things shifted. Tidal forces, or the gravitational pull our Moon exerts on our waters, started to reduce the rotation speed of the Earth, subsequently extending the day duration. 

The Sun's Gravitational Pull

Other forces also determine the Earth's day length. The Sun, in particular, has a significant gravitational pull on the atmospheric gas masses, which primarily causes atmospheric tides. Contrarily to the Moon slowing Earth's rotation, the Sun speeds it up.

The 19-Hour Day

The prediction is that Earth's days will continue to lengthen. The Moon, being the closest celestial body, exerts the most influence and is gradually moving away from Earth at a rate of 3.78 centimeters per year, adding 1.7 milliseconds per century to the day's length.

However, about two billion years ago, the Earth's day length stabilized at 19.5 hours, and only began to expand again 600 million years ago. The team of astrophysicists at the University of Toronto attribute this phenomenon to the balance achieved between the Moon's braking effect and the Sun's accelerating one. Over that long period, the Sun's effect on Earth's rotational speed increased due to elevated atmospheric gas temperatures, further driven by the synchronization of tides with atmospheric resonance.

Source: Wired