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] 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] a speed at which an object could circumnavigate the Earth's equator in 2 minutes and 54 seconds; that speed corresponds to approximately one 1300th 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]
The following list assumes that 1 galactic year is 225 million years.
| 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 |
| 6.666 galactic years ago | Eukaryotes appear |
| 6.8 galactic years ago | Multicellular organisms appear |
| 2.4 galactic years ago | Cambrian explosion occurs |
| 2 galactic years ago | The first brain structure appeared in worms |
| 1.11 galactic year ago | Permian–Triassic extinction event |
| 0.2935 galactic years ago | Cretaceous–Paleogene extinction event |
| 0.001 galactic years ago | Appearance of modern humans |
| Present day | |
|---|---|
| 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[5] |
| 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[6] |
| 12 galactic years from now | The Earth's magnetic field shuts down [7] and charged particles emanating from the Sun gradually deplete the atmosphere [8] |
| 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 will complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda [9] |
| 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 [10] |
| 2000 galactic years from now | Median Point of the Local Group of 47 galaxies[11] will coalesce into a single large galaxy |
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