Galactic year

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.

Approximate orbit of the Sun (yellow circle) around the Galactic Center

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]

Timeline of the universe and Earth's history in galactic years

Main articles: Timeline of the early universe, Timeline of natural history, and Timeline of the far future

The orientation of the Solar System's motion

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]

Visualization of the orbit of the Sun (yellow dot and white curve) around the Galactic Center (GC) in the last galactic year. The red dots correspond to the positions of the stars studied by the European Southern Observatory in a monitoring program.^[16]

See also

• Galactic Tick Day
• Geologic time scale

References

1. Cosmic Year Archived 2014-04-12 at the Wayback Machine, Fact Guru, University of Ottawa
2. Leong, Stacy (2002). "Period of the Sun's Orbit around the Galaxy (Cosmic Year)". The Physics Factbook.
3. http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000
4. Geologic Time Scale – as 18 galactic rotations
5. El Albani, Abderrazak; Bengtson, Stefan; Canfield, Donald E.; Riboulleau, Armelle; Rollion Bard, Claire; Macchiarelli, Roberto; et al. (2014). "The 2.1 Ga Old Francevillian Biota: Biogenicity, Taphonomy and Biodiversity". PLOS ONE. 9 (6): e99438. Bibcode:2014PLoSO...999438E. doi:10.1371/journal.pone.0099438. PMC 4070892. PMID 24963687.
6. El Albani, Abderrazak; Bengtson, Stefan; Canfield, Donald E.; Bekker, Andrey; Macchiarelli, Roberto; Mazurier, Arnaud; Hammarlund, Emma U.; et al. (2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago" (PDF). Nature. 466 (7302): 100–104. Bibcode:2010Natur.466..100A. doi:10.1038/nature09166. PMID 20596019. S2CID 4331375.^{[permanent dead link‍]}
7. F. M. Gradstein (2012). The geologic time scale 2012. Volume 2 (1st ed.). Amsterdam: Elsevier. ISBN 978-0-444-59448-8. OCLC 808340848.
8. Lunar and Planetary Institute (2010), https://www.lpi.usra.edu/features/chicxulub/
9. Williams, Caroline; Nield, Ted (2007-10-17). "Pangaea, the comeback". New Scientist. Retrieved 2014-01-02.
10. Franck, S.; Bounama, C.; von Bloh, W. (2005-11-07). "Causes and timing of future biosphere extinction". Biogeosciences Discussions. 2 (6). Copernicus GmbH: 1665-1679. Bibcode:2006BGeo....3...85F. doi:10.5194/bgd-2-1665-2005. S2CID 3619702.
11. Cox, T. J.; Loeb, Abraham (2008-05-01). "The collision between the Milky Way and Andromeda". Monthly Notices of the Royal Astronomical Society. 386 (1). Oxford University Press (OUP): 461–474. arXiv:0705.1170. Bibcode:2008MNRAS.386..461C. doi:10.1111/j.1365-2966.2008.13048.x. ISSN 0035-8711. S2CID 14964036.
12. Loeb, Abraham (2011-04-18). "Cosmology with hypervelocity stars". Journal of Cosmology and Astroparticle Physics. 2011 (4). IOP Publishing: 023. arXiv:1102.0007. Bibcode:2011JCAP...04..023L. doi:10.1088/1475-7516/2011/04/023. ISSN 1475-7516. S2CID 118750775.
13. Frommert, Hartmut; Kronberg, Christine (2007-06-05). "The Local Group of Galaxies". University of Arizona. Students for the Exploration and Development of Space. Archived from the original on 2009-09-07. Retrieved 2009-10-02.
14. Adams, Fred C.; Laughlin, Gregory (1997-04-01). "A dying universe: the long-term fate and evolutionof astrophysical objects". Reviews of Modern Physics. 69 (2): 337–372. arXiv:astro-ph/9701131. Bibcode:1997RvMP...69..337A. doi:10.1103/revmodphys.69.337. ISSN 0034-6861. S2CID 12173790.
15. "Milky Way Past Was More Turbulent Than Previously Known". ESO News. European Southern Observatory. 2004-04-06. After more than 1,000 nights of observations spread over 15 years, they have determined the spatial motions of more than 14,000 solar-like stars residing in the neighborhood of the Sun.
16. "Milky Way Past Was More Turbulent Than Previously Known". ESO News. European Southern Observatory. 2004-04-06. After more than 1,000 nights of observations spread over 15 years, they have determined the spatial motions of more than 14,000 solar-like stars residing in the neighborhood of the Sun.