One Day on Earth Wasn’t Always 24 Hours Long

When Homo sapiens began walking the Earth some 400,000 years ago, a day was basically 24 hours long — but that hasn’t always been the case. Scientists from Kyoto University estimate that when the moon first formed a few billion years ago, it spun around the Earth at a much closer distance than it does today, which affected the Earth’s own rotation. By their calculations, when life first appeared 3.6 billion years ago, an Earth day (one full rotation of the planet) was only 12 hours long. As the moon slowly distanced itself from Earth, the days grew longer, lasting 18 hours around the emergence of photosynthesis and 23 hours when multicellular life first took form. Research in 2021 discovered that the Earth is now spinning ever-so-slightly faster than it was 50 years ago, a major headache for physicists, astronomers, and computer programmers everywhere.