Thursday, July 2, 2015

Devonian - long years, short days

The Devonian covers the Earth's history from 419 to 359 million years ago. The world was a little different back then. The picture above shows the continents at the time. I find it essentially unrecognizable.

At the time, days lasted about 21 hours, but there were about 400 of them each year. I don't know how exact those numbers are, but that indicates 8400 hours a year compared to 8766 hours a year now. My guess is that the 400 is a rough estimate. I wasn't able to find a wikipedia article on deviation of the length of the Earth's revolution about the sun, but some quick googling turned up discussions about how the revolution about the sun is slowing, but at a very slow rate compared to changes in Earth's rotation.

The Earth's rotation is slowing over time, largely due to the dragging effect of the Moon's tides. However, there are occasional events like the 2004 Indian Ocean earthquake which increased Earth's rotation by a full 3 microseconds.

On a side note, it's fascinating that the historical view that the heavens rotate about the Earth was not without controversy. Even in the time of the ancient Greeks there were some who argued that the Earth rotated. The counterargument was that if the Earth rotated, the world would be destroyed by high winds.

The Devonian had its share of extinction intensity increases.

At the beginning of the Devonian the largest land organism was a fungus that stood 8 meters tall. By the middle of the Devonian plants developed roots and leaves, so now the land looked something like this:

The following shows the evolution of animals over geologic time. Note that the Devonian is the age of the fishes. According to this graph, before that period there wasn't nearly as much variety in the oceans. The end of the Devonian is the rise of the tetrapods.

Time goes from the bottom to the top in that graph. The width of each line shows the preponderance of that form of life during that period of time.

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