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 convenient medial 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 and Timeline of the far future. The following list assumes that 1 galactic year is 225 million years.
| Time | Event | ||
|---|---|---|---|
| Galactic years (gal) (approx.) | Millions of years (Ma) (approx.) | ||
| Past (years ago) | |||
| 61 gal | 13725 Ma (13.7 Ga) | Big Bang | |
| 60 gal | 13500 Ma (13.5 Ga) | Birth of the Milky Way | |
| 49 gal | 11025 Ma (11 Ga) | A hypothesized merge of Milky Way with Kraken galaxy[5] [6] | |
| 20 gal | 4500 Ma | Birth of the Sun and Earth[7] | |
| 17–18 gal | 3825–4050 Ma | Oceans appear on Earth | |
| 17 gal | 3825 Ma | Life begins on Earth | |
| 16 gal | 3600 Ma | Prokaryotes appear | |
| 12 gal | 2700 Ma | Bacteria appear | |
| 11 gal | 2475 Ma | The Great Oxidation Event commences[8] | |
| 10 gal | 2250 Ma | Eukaryian period[9] [10] first appearance of eukaryotes[11] Stable continents appear | |
| 7 gal | 1575 Ma | Multicellular organisms appear | |
| 5 gal | 1125 Ma | Meiosis and sexual reproduction appear[12] | |
| 4 gal | 900 Ma | First multicellular terrestrial plants[13] | |
| 3 gal | 675 Ma | Possible early animals (Animalia)[14] [15] | |
| 2 gal | 540 Ma | Cambrian explosion occurs | |
| 2 gal | 500 Ma | The first brain structure appears in worms | |
| 1 gal | 225 Ma | Permian–Triassic extinction event | |
| 0.3 gal | 68 Ma | Cretaceous–Paleogene extinction event | |
| 0.001 gal | 0.23 Ma | Emergence of anatomically modern humans | |
| Future (years from now) | |||
| 0.15 gal | 34 Ma | Mean time between impacts of asteroidal bodies in the order of magnitude of the K/Pg impactor has elapsed.[16] | |
| 1 gal | 225 Ma | All the continents on Earth may fuse into a supercontinent. Three potential arrangements of this configuration have been dubbed Amasia, Novopangaea, and Pangaea Proxima.[17] | |
| 2–3 gal | 450–675 Ma | Tidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible | |
| 4 gal | 900 Ma | Carbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out[18] | |
| 15 gal | 3375 Ma | Surface conditions on Earth are comparable to those on Venus today | |
| 22 gal | 4950 Ma | The Milky Way and Andromeda Galaxy begin to collide | |
| 25 gal | 5625 Ma | Sun ejects a planetary nebula, leaving behind a white dwarf | |
| 30 gal | 6750 Ma | The Milky Way and Andromeda complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda[19] | |
| 500 gal | 112500 Ma (112.5 Ga) | The Universe's expansion causes all galaxies beyond the Milky Way's Local Group to disappear beyond the cosmic event horizon, removing them from the reachable universe[20] | |
| 2000 gal | 450000 Ma (450 Ga) | Local Group of 47 galaxies[21] coalesces into a single large galaxy[22] | |
