A cosmic year is the time it takes for the Sun to complete one orbit around the center of the Milky Way. There are hundreds of billions of stars in our galaxy and they are all rotating around an incredibly dense gravitational force at its center. The gravitational mass is over 100 billion times the mass of our sun. Every star does not orbit at the same rate with the stars closer to the center move at a faster rate than the stars further from the center. Our star is located approximately 30,000 light years from the center while the entire galaxy is over 100,000 light years across.

To calculate the speed at which we travel around the center of the galaxy, astronomers calculate the distance we are from the gravitational center and the speed at which we are orbiting, 220 km/s (or 1/1300 the speed of light) to determine the length of time necessary to complete one revolution. This is approximately 220 million years. This is known as a cosmic year. According to modern astronomers our sun has completed this trip around the center of the Milky Way more than 20 times in its 5 billion year history. The motions of the other galactic bodies are studied by measuring the positions of lines in the galaxy spectra.

In cosmic terms the entire universe, as we currently understand it, is only just over 61 cosmic years old. In cosmic years the first forms of life appeared about 17 years ago. Dinosaurs were wiped out about .03 years ago and humans made their appearance .001 years ago.

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