The science behind Earth’s 420-day years

In Earth’s distant past, a year lasted not 365 days, but 420 — not because Earth’s orbit was longer, but because the planet spun faster, resulting in shorter days. Over time, as Earth's rotation slowed, the number of days in a year decreased. Scientists now believe that during the late Dinosaur Era, a year had around 372 days, and when the planet first formed, it may have fit over 500 days into a single orbit.

The slowing of Earth’s spin is largely attributed to the Moon’s influence. Though not the largest satellite in the Solar System, the Moon is unusually large in comparison to Earth, giving it a powerful gravitational pull. This interaction causes tides — both in the oceans and in Earth’s crust — which create friction. That friction, in turn, leads to a slight delay in Earth's tidal bulge, according to an article by IFLScience. 

Because the bulge isn’t perfectly aligned with the Earth-Moon axis, it exerts a torque on the Moon, pushing it farther away. To conserve angular momentum within the Earth-Moon system, Earth’s rotation slows as the Moon moves outward.

As days lengthen, fewer of them fit into a year. But determining how many days made up a year at specific points in Earth’s history remains a scientific challenge. Early estimates suggest that 1.4 billion years ago, Earth had 18-hour days and roughly 490 days in a year. However, newer research proposes a much more stable rotation from 2 to 1 billion years ago, suggesting a year of around 460 days during that era — possibly due to the Sun’s impact on Earth’s atmosphere counteracting the Moon’s slowing effect.

Clues from ancient corals have helped scientists refine these estimates. In 1963, Cornell University’s Professor John Wells published a seminal study analyzing growth patterns in fossilized corals from the Middle Devonian Period, around 380 million years ago.

These corals revealed a 400-day year, plus or minus a week. Similar studies of Silurian Period corals, dating from 444 to 419 million years ago, found years spanning 400 to 420 days. This upper limit — 420 — has since become the most commonly cited figure in public discourse.

However, not all scientists agree. In 2000, Dr. George Williams of the University of Adelaide examined even older coral samples and estimated 400 days in a year around 620 million years ago. He also produced radically different figures — including 466 and 514 days per year — from samples dating back 2.45 billion years, using alternate methods.

With reputable researchers reaching widely differing conclusions, there’s no single accepted timeline for exactly when Earth experienced 420-day years. But the scientific consensus remains clear: Earth’s days were once shorter, and there were many more of them in a year.

While the exact number at specific points in history remains debated, one thing is certain — our planet’s rotation, and the number of days it fits into a year, has changed dramatically over time.

By Sabina Mammadli