A day has not always been 24 hours long. In fact, it began lasting only 4 hours. The reasons for this extreme variation were explained by planetary scientist Takanori Sasaki, from the Kyoto University, during the Physics Workshop of the second phase of the Intercontinental Academia (ICA), on March 9.

Sasaki said that the formation of the Earth and the Moon, 4.5 billion years ago, and the influence of the Moon on the planet are the determinants of the length variation of a day and a month throughout the Earth's history.

Planetary scientist Takanori Sasaki

According to him, the most accepted hypothesis to explain the origin of the Moon is the occurrence of a giant impact between a Mars-sized body and what could be called the proto-Earth.

But when did this impact occur exactly? Sasaki explained that to have this question answered researchers analyze the transformation of the isotope hafnium-182 into the isotope tungsten-182. "Hafnium is a lithophile (rock-loving) element and tungsten is a siderophile (iron-loving) element, respectively connected to the mantle and the core of a star.

According to Sasaki, the giant impact has produced a magma ocean on the proto-Earth, which seems to have lead to a considerable separation between metal and silicates. Thus, the age of the hafnium-tungsten (Hf-W) separation would be the age of the last huge impact, that is, the age of the Earth and the Moon. "It is possible to calculate how much tungsten the mantle has and thus determine the age of the planet." Using this method, it has been concluded that the Earth and the Moon emerged at the beginning of the solar system, 62 million years after the system's rise, 4.5 billion years ago.

The impact has generated a large number of fragments around the Earth, which then regrouped giving rise to the Moon at an orbit just above the Roche limit (minimum distance from the center of the planet that a satellite can orbit without being destroyed by the severity of the tidal forces), said Sasaki. This limit is at a distance of three times the Earth's radius, but now the Moon is at a distance of 60 times the radius size, and should stop to move away when the distance reaches 80 times the radius size, in a multibillion years.

To measure the distance between the Earth and the Moon scientists use time: how long it takes for a laser beam to reach the Moon, be reflected and reach the Earth. The Lunar Laser Ranging Experiment uses this method and the first measurement was made in 1969. With this method, it was decided that the Moon is at 384,400 km from the Earth. Then, the Experiment found a surprising fact: analyzing the data from January 1992 to April 2001, the researchers found that the Moon is moving away 3.8 cm per year. "If this is correct, then the Moon was much closer in the past," Sasaki said.

There is an exchange of angular momentum between the Moon and the Earth. Sasaki cited a hypothesis that is mentioned in the reference book of this area, "Solar System Dynamics," by Carl Murray and Stanley Dermott: "It is highly likely that the orbit of the Moon and the Earth's rotation have considerably changed during the existence of the solar system, especially due to the action of semidiurnal tides [when the Moon is over a location on the Earth and then on its opposite side] caused by the Moon to the Earth."

This means that the Moon attracts the mass of water and this reduces the speed of the Earth's rotation. At the same time, the tide shifting due to the Earth's rotation attracts the Moon, gaining angular momentum and gradually distancing. The Moon also gets slower, reducing the duration of the month.

Sasaki explained that according to the Kepler's 3rd Law (the square of the orbital period of a planet is directly proportional to the cube of half the major axis of its orbit), the closer to the Sun, the higher the speed of a planet, and the further away, the slower. This also applies to the Moon-Earth system.

An attempt to prove the variation in month length has been made by two researchers that studied the structure of a certain type of sea shell. For Sasaki, "this is a controversial article, but it provides some interesting directions." The shells develop lines of daily growth in segments with monthly growth. Analyzing shells today, it appears that they have 30 rows per segment, which means a 30-day month. "In fossil shells of 400 million years ago there are only 9 lines per segment, assuming that the month lasted 9 days. This indicates that the Moon spun faster around the Earth and at a distance 40% smaller than the current one."

After all, how long did a day last when the Earth and the Moon came to be? "At first, the Moon was at a distance of three times the Earth's radius, immediately after the Roche limit. With this distance and the estimated angular momentum, it can be said that the day lasted only 4 hours. Over time, the Moon moved away and the length of the day increased: when the planet and its satellite were 30,000 years old, the day lasted six hours; when they were 60 million years old, the day lasted 10 hours."

At the end of his presentation, Sasaki presented a graph relating the development of life ("though not an expert on the issue") with the length of the day through time. According to it, the first evidence of life, 3.5 billion years ago, happened when the day lasted 12 hours. The emergence of photosynthesis, 2.5 billion years ago, happened when the day lasted 18 hours. 1.7 billion years ago the day was 21 hours long and the eukaryotic cells emerged. The multicellular life began when the day lasted 23 hours, 1.2 billion years ago. The first human ancestors arose 4 million years ago, when the day was already very close to 24 hours long.