Today we take for granted that a day lasts 24 hours. We organize work, meals, sleep and much of our lives around that cycle. But for most of the planet’s history this was not the case. If the entire history of the Earth were compressed into one year, 24-hour days would not even occupy a week. For the vast majority of the calendar, our planet rotated faster than it does today. The Sun rose and set six times during what we today call a day. A time when dawn came every few hours and the Earth rotated so fast that completing a revolution on itself only took four hours.
That means that, if the Earth continued to rotate with four-hour days, a year would continue to last practically the same, but it would have about 2,190 sunrises instead of the current 365. Therefore, a ten-year-old child would have seen more sunrises than a sixty-year-old person in our world. The story begins about 4.5 billion years ago. By then, Earth had just survived a gigantic collision with a body roughly the size of Mars. From that impact the Moon would emerge. The violence of the crash left the young Earth spinning at breakneck speed.
To imagine it, just think that the current planet rotates at about 1,670 kilometers per hour at the equator. That early Earth rotated approximately six times as fast. But, if the Earth rotated so fast, why do days last 24 hours today? The main person responsible is the Moon. Our satellite not only illuminates the nights and causes the tides. It also acts as a giant gravitational brake.
Tides generate friction in the oceans. That friction steals a small amount of energy from the Earth’s rotation and transfers it to the lunar orbit. As a result, the Earth rotates more and more slowly and the Moon slowly moves away from us. The process is extremely slow. Currently the length of the day increases by just a few milliseconds per century.
But over billions of years those small changes add up. Scientists have partially reconstructed this history thanks to fossils, ancient corals, seashells and astronomical models. When the first forms of life appeared about 3.5 billion years ago, days lasted about 12 hours. By the time photosynthesis arose, the duration had increased to about 18 hours. About 1.7 billion years ago it was around 9 p.m. Multicellular life appeared when the day was already approaching 11 p.m.
And now comes the key question: How would our lives be affected if the Earth rotated six times faster? Surprisingly, not in the way you usually imagine. Many people think that we would feel a constant wind or a kind of push back. It wouldn’t happen like that.
Just as we do not feel the 107,000 kilometers per hour at which the Earth travels around the Sun, we do not directly perceive the rotation because we, the atmosphere and the oceans move with the planet. However, the effects would be enormous: at the equator, the centrifugal force would be much stronger. Our weight would decrease slightly. The oceans would tend to accumulate closer to the equator and the shape of the Earth would become even more flattened. The atmospheric patterns would be radically different.
What we would notice immediately would be the rhythm of the sky. The Sun would rise at six in the morning, two hours later it would be noon. Another two hours later night comes, barely two or three hours long. This complete cycle of light and darkness would be repeated six times in the time it takes for a day to pass today. Our biological clocks would have a considerable problem. Human circadian rhythms are adapted to cycles close to 24 hours. In fact, even if any external reference is removed, our internal clock usually oscillates around that value.
A planet with four-hour days would force evolution to find other strategies. Perhaps organisms would ignore the solar cycle and develop their own biological clocks. Maybe they would sleep several times during each rotation. Perhaps the alternation between day and night would have much less ecological importance. Life would find a solution, but it would hardly be the same planet: photosynthesis, stomatal opening, growth and flowering depend on internal clocks set to approximately 24 hours. With four-hour days many modern plants simply would not work.
But it wouldn’t just change biology. An Earth spinning six times faster would have different atmospheric dynamics: The Coriolis force (the phenomenon responsible for the direction of winds, ocean currents and the direction of rotation of hurricanes) would be much more intense. Thus, wind patterns would change, ocean currents would be different, and daytime and nighttime temperatures would fluctuate less because the ground would have less time to warm up or cool down. Evolution would end up responding to a completely different world.
The length of the day is “simply” the result of a long gravitational negotiation between the Earth and the Moon. For billions of years our planet has been slowly reducing its spin speed, like a top that loses momentum. We humans have appeared at a very specific moment in that history, we are (as we conceive ourselves right now), an accident caused by the Moon.