Galactic year

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. [ Astronomy Knowledge Base] ] Estimates of the length of one orbit range from 225 to 250 million "terrestrial" years. cite web|url=|work=The Physics Factbook|last=Leong|first=Stacy|year=2002|title=Period of the Sun's Orbit around the Galaxy (Cosmic Year)]

The galactic year provides a conveniently "graspable" unit for thinking about cosmic and geological time periods. (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] ]

=Timeline of History in Galactic Years ( In this list 1GY = 225 million years)=

*0 GY: Birth of the Sun
*4 GY: Oceans appear on Earth
*5 GY: Life begins
*6 GY: Prokaryotes appear
*7 GY: Bacteria appear
*10 GY: Stable continents appear
*13 GY: Eukaryotes appear
*16 GY: Multi-cellular organisms appear
*17.8 GY: Cambrian explosion
*19 GY: Great Dying
*19.6 GY: K–T extinction event
*19.999 GY: Apparition of modern humans
*20 GY: Present day

ee also

*List of unusual units of measurement


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