Galactic year
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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]
[edit] Timeline of History in Galactic Years (GY)
This timeline starts at the birth of the solar system. The present day is assumed to be at 20 GY.
- 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
[edit] See also
[edit] References
- ^ Astronomy Knowledge Base
- ^ Leong, Stacy (2002). Period of the Sun's Orbit around the Galaxy (Cosmic Year). The Physics Factbook.
- ^ Geologic Time Scale
