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.[1] Estimates of the length of one orbit range from 225 to 250 million "terrestrial" years. [2]
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.) [3]
Timeline of earth's history in galactic years
In this list, 1 galactic year (GY) = 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: Multicellular organisms appear
- 17.8 GY: Cambrian explosion
- 19 GY: Great Dying
- 19.6 GY: K–T extinction event
- 19.999 GY: Appearance of modern humans
- 20 GY: Present day
See also
References
- ^ Cosmic Year, Fact Guru, University of Ottawa
- ^ Leong, Stacy (2002). "Period of the Sun's Orbit around the Galaxy (Cosmic Year)". The Physics Factbook. http://hypertextbook.com/facts/2002/StacyLeong.shtml.
- ^ Geologic Time Scale - as 18 galactic rotations
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