Galactic year

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Milky Way Arms ssc2008-10.svg

Approximate orbit of the Sun (yellow circle) around the Galactic Centre

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 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,[3] 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.[4]

Timeline of universe's and earth's history in galactic years[edit]

The following list assumes that 1 galactic year is 225 million years.

about 61 galactic years ago Big Bang
about 54 galactic years ago Birth of the Milky Way
18.4 galactic years ago Birth of the Sun
17-18 galactic years ago Oceans appear on Earth
15 galactic years ago Life begins on Earth
14 galactic years ago Prokaryotes appear
13 galactic years ago Bacteria appear
10 galactic years ago Stable continents appear
7 galactic years ago Eukaryotes appear
6.8 galactic years ago Multicellular organisms appear
2.8 galactic years ago Cambrian explosion
1 galactic year ago Permian–Triassic extinction event
0.26 galactic years ago Cretaceous–Paleogene extinction event
0.001 galactic years ago Appearance of modern humans
Present day
22 galactic years in the future The Milky Way and Andromeda Galaxy begin to collide
25 galactic years in the future Sun ejects a planetary nebula, leaving behind a white dwarf

Sun in orbit around Galactic Centre.gif

Visualisation of the orbit of the Sun (yellow dot and white curve) around the Galactic Centre (GC) in the last galactic year. The red dots correspond to the positions of the stars studied by the European Southern Observatory in a monitoring programme.[5]

See also[edit]


  1. ^ Cosmic Year, Fact Guru, University of Ottawa
  2. ^ Leong, Stacy (2002). "Period of the Sun's Orbit around the Galaxy (Cosmic Year)". The Physics Factbook. 
  3. ^ NASA - StarChild Question of the Month for February 2000
  4. ^ Geologic Time Scale - as 18 galactic rotations
  5. ^ "Milky Way Past Was More Turbulent Than Previously Known". ESO News. European Southern Observatory. 2004-04-06. "After more than 1,000 nights of observations spread over 15 years, they have determined the spatial motions of more than 14,000 solar-like stars residing in the neighbourhood of the Sun."