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 approximately 225 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]
See main article: 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 | ||
|---|---|---|---|
| Galactic years (gal) | Millions of years (Ma) | ||
| Past (years ago) | |||
| About 61.32 gal | Big Bang | ||
| About 54 gal | Birth of the Milky Way | ||
| 20.44 gal | Birth of the Sun | ||
| 17–18 gal | 3937 Ma | Oceans appear on Earth | |
| 16.889 gal | 3800 Ma | Life begins on Earth | |
| 15.555 gal | 3500 Ma | Prokaryotes appear | |
| 12 gal | 2700 Ma | Bacteria appear | |
| 10 gal | 2250 Ma | Eukaryian period[5] [6] first appearance of eukaryotes[7] Stable continents appear | |
| 6.8 gal | 1530 Ma | Multicellular organisms appear | |
| 2.4 gal | 540 Ma | Cambrian explosion occurs | |
| 2 gal | 500 Ma | The first brain structure appears in worms | |
| 1.11 gal | 250 Ma | Permian–Triassic extinction event | |
| 0.2933 gal | Cretaceous–Paleogene extinction event | ||
| 0.0013 gal | Emergence of anatomically modern humans | ||
| Future (years from now) | |||
| 0.15 gal | Mean time between impacts of asteroidal bodies in the order of magnitude of the K/Pg impactor has elapsed.[8] | ||
| 1 gal | All the continents on Earth may fuse into a supercontinent. Three potential arrangements of this configuration have been dubbed Amasia, Novopangaea, and Pangaea Ultima.[9] | ||
| 2–3 gal | Tidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible | ||
| 4 gal | Carbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out[10] | ||
| 15 gal | Surface conditions on Earth are comparable to those on Venus today | ||
| 22 gal | The Milky Way and Andromeda Galaxy begin to collide | ||
| 25 gal | Sun ejects a planetary nebula, leaving behind a white dwarf | ||
| 30 gal | The Milky Way and Andromeda complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda[11] | ||
| 500 gal | 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[12] | ||
| 2000 gal | Local Group of 47 galaxies[13] coalesces into a single large galaxy[14] | ||