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galactic year
The galactic year, also known as a cosmic year, is the duration of time required for the Solar System to orbit once around the center of the Milky Way Galaxy.〔(Cosmic Year ), Fact Guru, University of Ottawa〕 Estimates of the length of one orbit range from 225 to 250 million terrestrial years. The Solar System is traveling at an average speed of 828,000 km/h (230 km/s) or 514,000 mph (143 mi/s) within its trajectory around the galactic center,〔http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000〕 which is about one 1300th of the speed of light—a speed at which an object could circumnavigate the Earth's equator in 2 minutes and 54 seconds.
The galactic year provides a conveniently usable unit for depicting cosmic and geological time periods together. By contrast, a "billion-year" scale does not allow for useful discrimination between geologic events, and a "million-year" scale requires some rather large numbers.〔(Geologic Time Scale – as 18 galactic rotations )〕
==Timeline of the universe and Earth's history in galactic years==
The following list assumes that 1 galactic year is 225 million years.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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