The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy.[1] One galactic year is 230 million Earth years.[2] The Solar System is traveling at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph) within its trajectory around the galactic center,[3] a speed at which an object could circumnavigate the Earth's equator in 2 minutes and 54 seconds; that speed corresponds to approximately 1/1300 of the speed of light.
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]
Main articles: Timeline of the early universe, Timeline of natural history, and Timeline of the far future |
The following list assumes that 1 galactic year is 225 million years.
Time | Event |
---|---|
Past | |
About 61.32 galactic years ago | Big Bang |
About 54 galactic years ago | Birth of the Milky Way |
20.44 galactic years ago | Birth of the Sun |
17–18 galactic years ago | Oceans appear on Earth |
16.889 galactic years ago | Life begins on Earth |
15.555 galactic years ago | Prokaryotes appear |
12 galactic years ago | Bacteria appear |
10 galactic years ago | Stable continents appear |
6.8 galactic years ago | Multicellular organisms appear |
6.666 galactic years ago | Eukaryotes appear |
2.4 galactic years ago | Cambrian explosion occurs |
2 galactic years ago | The first brain structure appears in worms |
1.11 galactic year ago | Permian–Triassic extinction event |
0.2933 galactic years ago | Cretaceous–Paleogene extinction event |
0.0013 galactic years ago | Emergence of anatomically modern humans |
Future | |
0.15 galactic year from now | Mean time between impacts of asteroidal bodies in the order of magnitude of the K/Pg impactor has elapsed.[5] |
1 galactic year from now | All the continents on Earth may fuse into a supercontinent. Three potential arrangements of this configuration have been dubbed Amasia, Novopangaea, and Pangaea Ultima.[6] |
2–3 galactic years from now | Tidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible |
4 galactic years from now | Carbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out[7] |
15 galactic years from now | Surface conditions on Earth are comparable to those on Venus today |
22 galactic years from now | The Milky Way and Andromeda Galaxy begin to collide |
25 galactic years from now | Sun ejects a planetary nebula, leaving behind a white dwarf |
30 galactic years from now | The Milky Way and Andromeda complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda[8] |
500 galactic years from now | The Universe's expansion causes all galaxies beyond the Milky Way's Local Group to disappear beyond the cosmic light horizon, removing them from the observable universe[9] |
2000 galactic years from now | Local Group of 47 galaxies[10] coalesces into a single large galaxy[11] |