South Africa

Sand in South Africa reveals how long a day was 3.2 billion years ago: around 13 hours.

Here lie some of Earth’s oldest traces of tides. Each time the tide came and went, it left a thin layer of sand. The layers grew alternately thicker and thinner in step with the changing tides, and over time they’ve turned to sandstone.

By counting the layers, researchers can see how many tides and days fitted into a month. From that, they’ve estimated how long a day was. 

Since then, the Moon, through the tides, has slowly braked the Earth, and the days have grown almost twice as long.