Our life is structured around the daily cycle of 24 hours. But there is no fundamental reason for this period. In fact, it was different in the past and will be different in the future. The impact that gave birth to the Moon spun up the Earth and the subsequent gravitational interaction between the Moon and the Earth dictated the evolution of the duration of a day since then. In other words, the duration of a day on an Earth-like exoplanet without a Moon might be very different. We should keep that in mind when hosting biological visitors from interstellar space. They might go to sleep in their guest bedroom on very different intervals than we do. The label ET on their alarm clock would not mean `Eastern Time’ but rather `Extraterrestrial Time’.
The Moon was likely spawned as a result of a giant impact on Earth by a Mars-size object, commonly called Theia, in the early history of Earth. The impact occurred 60–175 million years after the Earth was born, which was 4.57 billion years ago.
The energy released during the impact melted the Earth’s surface into a magma ocean. Recent computer simulations of the Moon forming impact, suggest that the Earth was spun up by the impact to about a four-hour day. As the Moon receded from Earth, the day was lengthened by the tidal transfer of angular momentum from the Earth’s spin to the Moon’s orbit.
Following the collision, Earth’s surface temperature reached about 2,300 degrees Kelvin (3,680°F), too hot for liquid water to support the chemistry of life as-we-know-it. It took hundreds of millions of years for Earth to cool significantly, eventually allowing the emergence of the Last Universal Common Ancestor (LUCA) of life between 4.09–4.33 billion years ago. When LUCA lived, a day lasted about 10 hours. When photosynthesis started about 3.5 billion years ago, a day lasted 12 hours. By 1.8 billion years ago when complex organisms began, the day was 21 hours long. When the oldest known sexually reproducing organism formed, about 1.2 billion years ago, a day was about 20 hours. The first humans appeared a few million years ago, when the day was very close to the current 24 hours duration.
According to recent calculations, Earth’s spin is slowing down on average by 1.35 seconds every 100,000 years, or 3.75 hours in a billion years. Do other actors beyond the Moon have an effect on the Earth’s rotation?
Minor shifts in the duration of a terrestrial day occur all the time. For example, on June 29, 2022, Earth’s spin period was recorded to be 1.59 thousandths of a second (millisecond) shorter than 24 hours. Faster spin means that Earth gets to the same position earlier. Two milliseconds translate to a meter at the equator. GPS satellites could lose their precision if not corrected for such changes. There are related implications for cell phones, computers and communications systems, which synchronize with Network Time Protocol (NTP) servers.
Common changes reflect the motion of the Earth’s molten core, oceans, and atmosphere, seismic activity and the gravitational influences of the Moon and the Sun. Climate warming melts polar ice and adds liquid water at the equator, making the Earth less spherical. Similarly to the slower spin of ice skaters who extend their arms, angular momentum conservation implies that a more extended mass distribution than spherical causes Earth to spin slower as a result of climate warming.
Cataclysmic events also modify slightly the duration of the day on Earth. The most dramatic climate catastrophe is predicted to occur in a billion years when the Sun will boil off all water reservoirs on the Earth’s surface. As this calamity will shift 0.02% of the mass of the Earth into the atmosphere, it could increase the duration of a day by a tenth of a second. By that time, the day could be several hours longer than 24 hours as a result of the continuing interaction with the Moon.
Past climate catastrophes were less dramatic, The Chicxulub impactor which killed off 75% of all terrestrial species, including non-avian dinosaurs, changed the rotation energy of the Earth by at most a fraction of ten billionths and modified the duration of a day by less than a millisecond.
Dramatic earthquakes can decrease the length of day by a few millionths of a second (microseconds). The Three-Gorge reservoir in China holds 40 cubic kilometers of water. The shift of mass when it gets filled increases the length of day by 0.06 microseconds. The launch of rockets, including Starship, has a completely negligible effect on the duration of a terrestrial day.
In 7.6 billion years, the Sun will expand to a red giant and may engulf the Earth and the Moon. Once that happens, the Moon will spiral inwards and crash on Earth and the Earth’s rotation will slow down due to friction on the red giant’s envelope. Unfortunately, no terrestrial inhabitants will be able to get more sleep during the longer days at that time.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
