Coral lays down a layer of calcium carbonate every day. These layers can be counted, and so a thicker layer of 365 of them represent a year. Without knowing the length of a year, could we tell it's 365 days? Yes. The thickness of the layers varies with the seasonal variation in day length so there is a cyclic pattern to the layers. [2]
What about coral fossils? What do they tell us? Looking at fossils from 400 million years ago, there are about 410 layers per year. [1] What does this mean? Since the time it takes Earth to orbit the Sun is constant, it means that the length of a day was shorter, about 21 hours long [1,2]. Hence the earth is now spinning more slowly than it was 400 million years ago. Extrapolating back to 4.5 billion years ago, when a massive collision caused the formation of the moon [3], each day would only last about 6 hours.[1]
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