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.Cosmic Year , Fact Guru, University of Ottawa One galactic year is approximately 225 million Earth years. 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,http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000 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.Geologic Time Scale – as 18 galactic rotations
Timeline of the universe and Earth's history in galactic years
The following list assumes that one galactic year is 225 million years. TimeEventGalacticyears(gal) (approx.)Millions of years (Ma) (approx.)Past (years ago)61 gal 13725 Ma (13.7 Ga)Big Bang60 gal 13500 Ma (13.5 Ga)Birth of the Milky Way49 gal 11025 Ma (11 Ga)A hypothesized merge of Milky Way with Kraken galaxy 20 gal 4500 MaBirth of the Sun and EarthDalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Geological Society, London, Special Publications. 190 (1): 205–221. 17–18 gal 3825–4050 MaOceans appear on Earth17 gal 3825 MaLife begins on Earth16 gal 3600 MaProkaryotes appear12 gal 2700 MaBacteria appear11 gal 2475 MaThe Great Oxidation Event commencesBuick, Roger (August 27, 2008). "When did oxygenic photosynthesis evolve?". Philosophical Transactions of the Royal Society B. 363 (1504): 2731–2743. 10 gal 2250 MaEukaryian period first appearance of eukaryotes Stable continents appear7 gal 1575 MaMulticellular organisms appear5 gal 1125 MaMeiosis and sexual reproduction appearBernstein, Harris; Bernstein, Carol; Michod, Richard E. (2012). "DNA Repair as the Primary Adaptive Function of Sex in Bacteria and Eukaryotes". In Kimura, Sakura; Shimizu, Sora (eds.). DNA Repair: New Research. Hauppauge, NY: Nova Science Publishers. ISBN 978-1-62100-808-8. LCCN 2011038504. OCLC 828424701.4 gal 900 MaFirst multicellular terrestrial plants3 gal 675 MaPossible early animals (Animalia)2 gal 540 MaCambrian explosion occurs2 gal 500 MaThe first brain structure appears in worms1 gal 225 MaPermian–Triassic extinction event0.3 gal 68 MaCretaceous–Paleogene extinction event0.001 gal 0.23 MaEmergence of anatomically modern humansFuture (years from now)0.15 gal34 MaMean time between impacts of asteroidal bodies in the order of magnitude of the K/Pg impactor has elapsed.Lunar and Planetary Institute (2010), https://www.lpi.usra.edu/features/chicxulub/1 gal225 MaAll the continents on Earth may fuse into a supercontinent. Four potential arrangements of this configuration have been dubbed Amasia, Novopangaea, Aurica and Pangaea Proxima.2–3 gal450–675 MaTidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible4 gal900 MaCarbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out15 gal3375 MaSurface conditions on Earth are comparable to those on Venus today22 gal 4950 MaThe Milky Way and Andromeda Galaxy begin to collide25 gal 5625 MaSun ejects a planetary nebula, leaving behind a white dwarf30 gal 6750 MaThe Milky Way and Andromeda complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda500 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 universe2000 gal 450000 Ma (450 Ga)Local Group of 47 galaxies coalesces into a single large galaxy
See also
Galactic Tick Day Geologic time scale
References