Galactic_year - Wikiparency

# Galactic year  **Microsim — three.js (Wikitube framework):** *Out to the nearest star (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=neighborhood&embed=1" data-title="Out to the nearest star (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=neighborhood`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: follow the ruler from the heliopause tick at 121 AU to Proxima Centauri at 4.25 light-years, the largest distance the explorer draws; set show to clouds and boundaries to see the Oort cloud points and the boundary shells on their own; press l to hide the labels and drag the view edge-on, and remember that one galactic year carries the whole scene, ruler and all, once around the Milky Way.* A **galactic year**, also called a **cosmic year**, is the time the [[Sun]] takes to travel once around the centre of the [[Milky_Way|Milky Way]]. It is about 225 million years.[^leong2002] The Sun moves along its galactic [[Orbit|orbit]] at roughly 230 km/s; at that speed it would circle [[Earth]]'s equator, 40,075 km, in about 2 minutes 54 seconds, and it covers about 1/1,300 of the speed of light (both derived).[^starchild] The period follows from two measured quantities, the Sun's distance from the Galactic Centre and the circular speed of rotation there. Reid and colleagues find 8.15 kiloparsecs and 236 km/s from the parallaxes and motions of star-forming regions, which gives 2π × 8.15 kpc ÷ 236 km/s ≈ 212 million years (derived); following stars around the central black hole gives a distance of 8.32 kiloparsecs, and textbook values of the galactic year range up to about 240 million years.[^reid2019][^gillessen2017][^sparke2007] The Sun's actual path, set by the [[Gravity|gravity]] of the whole Galaxy, is not a closed circle: it oscillates above and below the galactic plane and is perturbed by the spiral arms, so any galactic year is an average.[^sparke2007] Because the unit sits between the million-year and billion-year scales, it is a convenient way to place cosmic and geological history side by side. The explorer at the top of this page cannot show such an orbit. Its neighbourhood view is a ruler that stops at Proxima Centauri, 4.25 light-years out, while one galactic circuit is about 168,000 light-years long (derived from 2π × 8.15 kpc). ## Timeline of the universe and Earth's history in galactic years The table below uses 225 million years per galactic year, rounded; the dates come from the sources cited on each row. With this unit the [[Formation_and_evolution_of_the_Solar_System|Solar System]] is about 20 galactic years old and the dinosaurs died out about 0.3 of a galactic year ago, when the Sun was roughly 105° back along its orbit (derived from 66/225 × 360°). | Galactic years | Millions of years | Event | Source | |---|---|---|---| | 61 ago | 13,800 ago | Big Bang | Planck 2018[^planck2020] | | 49 ago | about 11,000 ago | Merger of the young Milky Way with the "Kraken" galaxy (from simulations) | Kruijssen et al. 2020[^kruijssen2020] | | 20 ago | 4,540 ago | Formation of the Sun and Earth | Dalrymple 2001[^dalrymple2001] | | 11 ago | about 2,450 ago | Great Oxidation Event under way | Buick 2008[^buick2008] | | 9 ago | 2,100 ago | Large colonial organisms in oxygenated seas (Francevillian biota) | El Albani et al. 2010[^elalbani2010] | | 4.5 ago | about 1,000 ago | Earliest non-marine eukaryotes | Strother et al. 2011[^strother2011] | | about 3 ago | about 650 ago | Possible earliest animals, by molecular clocks and fossils | Cunningham et al. 2017[^cunningham2016] | | 1.1 ago | 251.9 ago | Permian–Triassic mass extinction | Burgess et al. 2014[^burgess2014] | | 0.29 ago | 66.0 ago | Cretaceous–Paleogene extinction | Renne et al. 2013[^renne2013] | | 0.0014 ago | about 0.3 ago | Earliest *Homo sapiens* fossils | Hublin et al. 2017[^hublin2017] | | 1 from now | about 250 from now | Continents may merge into a new supercontinent | Williams and Nield 2007[^williams2007] | | 4 from now | about 900 from now | Falling carbon dioxide ends most multicellular life | Franck et al. 2005[^franck2005] | | 17–18 from now | about 3,900 from now | First close passage of the Andromeda Galaxy | van der Marel et al. 2012[^vandermarel2012] | | 26 from now | about 5,900 from now | Milky Way and Andromeda merged into one galaxy | van der Marel et al. 2012[^vandermarel2012] | | about 31–36 from now | about 7,000–8,000 from now | Sun ends as a white dwarf after its giant phases | Sackmann et al. 1993[^sackmann1993] | | about 440 from now | about 100,000 from now | Galaxies beyond the Local Group pass beyond the cosmic horizon | Loeb 2011[^loeb2011] | Galactic-year values are the million-year figures divided by 225 and rounded (derived). Read this way, the long history of life's [[Evolution|evolution]] is compressed into a few circuits. [[Earth]] formed about 20 galactic years ago, and [[Oxygen|oxygen]] began to build up in its [[Atmosphere_of_Earth|atmosphere]] about 11 galactic years ago, after oxygen-producing [[Photosynthesis|photosynthesis]] had evolved.[^buick2008] Complex multicellular life belongs to the last few circuits: the Francevillian fossils are about 9 galactic years old, and molecular clocks and fossils place the origin of animals about 3 galactic years back.[^elalbani2010][^cunningham2016] The two largest mass extinctions of the last half-billion years, at the end of the Permian and the end of the Cretaceous, are one and about a third of a galactic year ago.[^burgess2014][^renne2013] All of human history fits into roughly one-thousandth of a galactic year: since the earliest known *Homo sapiens*, the Sun has moved only about half a degree around the Galaxy (derived from 0.3/225 × 360°).[^hublin2017] The future scale is set by stellar and galactic evolution. In about 4 galactic years, the slow brightening of the Sun and falling carbon dioxide are expected to end most multicellular life on Earth.[^franck2005] The Andromeda Galaxy should first pass close to the Milky Way in about 17 galactic years and merge with it by about 26.[^vandermarel2012] Several billion years from now the Sun will exhaust the [[Hydrogen|hydrogen]] in its core, swell into a red giant and end, some 31 to 36 galactic years from now, as a white dwarf.[^sackmann1993] On far longer scales, the expansion of the universe will carry every galaxy outside the Local Group beyond view, and the Local Group itself is expected eventually to coalesce into a single large galaxy.[^loeb2011][^adams1997] ## See also - [[Milky_Way]] - [[Orion_Arm]] - [[Formation_and_evolution_of_the_Solar_System]] - [[Local_Bubble]] - Geologic time scale · Galactic Tick Day ## References [^leong2002]: Leong, S. (2002). "Period of the Sun's orbit around the Galaxy (cosmic year)". In Elert, G. (ed.), *The Physics Factbook*. http://hypertextbook.com/facts/2002/StacyLeong.shtml [^starchild]: NASA Goddard Space Flight Center (February 2000). "StarChild question of the month: does the Sun move around the Milky Way?". https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html [^reid2019]: Reid, M. J.; Menten, K. M.; Brunthaler, A.; et al. (2019). "Trigonometric parallaxes of high-mass star-forming regions: our view of the Milky Way". *The Astrophysical Journal* 885: 131. https://doi.org/10.3847/1538-4357/ab4a11 [^gillessen2017]: Gillessen, S.; Plewa, P. M.; Eisenhauer, F.; et al. (2017). "An update on monitoring stellar orbits in the Galactic Center". *The Astrophysical Journal* 837: 30. https://doi.org/10.3847/1538-4357/aa5c41 [^sparke2007]: Sparke, L. S.; Gallagher, J. S. (2007). *Galaxies in the Universe: An Introduction*, 2nd ed. Cambridge University Press, p. 90. ISBN 978-1-139-46238-9. [^planck2020]: Planck Collaboration; Aghanim, N.; et al. (2020). "Planck 2018 results. VI. Cosmological parameters". *Astronomy & Astrophysics* 641: A6. https://doi.org/10.1051/0004-6361/201833910 [^kruijssen2020]: Kruijssen, J. M. D.; Pfeffer, J. L.; Chevance, M.; et al. (2020). "Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations". *Monthly Notices of the Royal Astronomical Society* 498: 2472–2491. https://doi.org/10.1093/mnras/staa2452 [^dalrymple2001]: Dalrymple, G. B. (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". *Geological Society, London, Special Publications* 190: 205–221. https://doi.org/10.1144/GSL.SP.2001.190.01.14 [^buick2008]: Buick, R. (2008). "When did oxygenic photosynthesis evolve?". *Philosophical Transactions of the Royal Society B* 363: 2731–2743. https://doi.org/10.1098/rstb.2008.0041 [^elalbani2010]: El Albani, A.; Bengtson, S.; Canfield, D. E.; et al. (2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago". *Nature* 466: 100–104. https://doi.org/10.1038/nature09166 [^strother2011]: Strother, P. K.; Battison, L.; Brasier, M. D.; Wellman, C. H. (2011). "Earth's earliest non-marine eukaryotes". *Nature* 473: 505–509. https://doi.org/10.1038/nature09943 [^cunningham2016]: Cunningham, J. A.; Liu, A. G.; Bengtson, S.; Donoghue, P. C. J. (2017). "The origin of animals: can molecular clocks and the fossil record be reconciled?". *BioEssays* 39: 1–12. https://doi.org/10.1002/bies.201600120 [^burgess2014]: Burgess, S. D.; Bowring, S.; Shen, S. (2014). "High-precision timeline for Earth's most severe extinction". *Proceedings of the National Academy of Sciences* 111: 3316–3321. https://doi.org/10.1073/pnas.1317692111 [^renne2013]: Renne, P. R.; Deino, A. L.; Hilgen, F. J.; et al. (2013). "Time scales of critical events around the Cretaceous-Paleogene boundary". *Science* 339: 684–687. https://doi.org/10.1126/science.1230492 [^hublin2017]: Hublin, J.-J.; Ben-Ncer, A.; Bailey, S. E.; et al. (2017). "New fossils from Jebel Irhoud, Morocco and the pan-African origin of *Homo sapiens*". *Nature* 546: 289–292. https://doi.org/10.1038/nature22336 [^williams2007]: Williams, C.; Nield, T. (17 October 2007). "Pangaea, the comeback". *New Scientist*. https://www.newscientist.com/article/mg19626261-500-pangaea-the-comeback/ [^franck2005]: Franck, S.; Bounama, C.; von Bloh, W. (2005). "Causes and timing of future biosphere extinction". *Biogeosciences Discussions* 2: 1665–1679. https://doi.org/10.5194/bgd-2-1665-2005 [^vandermarel2012]: van der Marel, R. P.; Fardal, M.; Besla, G.; et al. (2012). "The M31 velocity vector. II. Radial orbit toward the Milky Way and implied Local Group mass". *The Astrophysical Journal* 753: 8. https://doi.org/10.1088/0004-637X/753/1/8 [^sackmann1993]: Sackmann, I.-J.; Boothroyd, A. I.; Kraemer, K. E. (1993). "Our Sun. III. Present and future". *The Astrophysical Journal* 418: 457–468. https://doi.org/10.1086/173407 [^loeb2011]: Loeb, A. (2011). "Cosmology with hypervelocity stars". *Journal of Cosmology and Astroparticle Physics* 2011 (04): 023. https://doi.org/10.1088/1475-7516/2011/04/023 [^adams1997]: Adams, F. C.; Laughlin, G. (1997). "A dying universe: the long-term fate and evolution of astrophysical objects". *Reviews of Modern Physics* 69: 337–372. https://doi.org/10.1103/RevModPhys.69.337 ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Galactic_year) : [Wikitube](https://en.wikitube.io/wiki/Galactic_year) · pinned revision [1374681327](https://en.wikipedia.org/w/index.php?oldid=1374681327) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-093 · explorer state `?view=neighborhood`.*