The galactic year, also known as a cosmic year, is the duration of time required for the Solar System to orbit once around the center of the Milky Way Galaxy. Estimates of the length of one orbit range from 225 to 250 million terrestrial years. 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, which is about one 1300th of the speed of light—a speed at which an object could circumnavigate the Earth's equator in 2 minutes and 54 seconds.
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.
Timeline of universe's and earth's history in galactic years
The following list assumes that 1 galactic year is 225 million years.
|about 61 galactic years ago||Big Bang|
|about 54 galactic years ago||Birth of the Milky Way|
|18.4 galactic years ago||Birth of the Sun|
|17–18 galactic years ago||Oceans appear on Earth|
|15 galactic years ago||Life begins on Earth|
|14 galactic years ago||Prokaryotes appear|
|13 galactic years ago||Bacteria appear|
|10 galactic years ago||Stable continents appear|
|7 galactic years ago||Eukaryotes appear|
|6.8 galactic years ago||Multicellular organisms appear|
|2.8 galactic years ago||Cambrian explosion|
|1 galactic year ago||Permian–Triassic extinction event|
|0.26 galactic years ago||Cretaceous–Paleogene extinction event|
|0.001 galactic years ago||Appearance of modern humans|
|1 galactic year in the future||All the continents on Earth may fuse into a supercontinent. Three potential
arrangements of this configuration have been dubbed Amasia, Novopangaea,
and Pangaea Ultima
|2–3 galactic years in the future||Tidal acceleration moves the Moon far enough from Earth that
total solar eclipses are no longer possible
|4 galactic years in the future||Carbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible.
Multicellular life dies out
|12 galactic years in the future||the Earth's magnetic field shuts down 
and charged particles emanating from the Sun gradually deplete the atmosphere 
|15 galactic years in the future||Surface conditions on Earth are comparable to those on Venus today|
|22 galactic years in the future||The Milky Way and Andromeda Galaxy begin to collide|
|25 galactic years in the future||Sun ejects a planetary nebula, leaving behind a white dwarf|
|30 galactic years in the future||the Milky Way and Andromeda will complete their merger
into a giant Elliptical galaxy called Milkomeda or Milkdromeda 
|500 galactic years in the future||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 
|2000 galactic years in the future||Median Point of which the Local Group of 47 galaxies will coalesce into a single large galaxy |
- Cosmic Year, Fact Guru, University of Ottawa
- Leong, Stacy (2002). "Period of the Sun's Orbit around the Galaxy (Cosmic Year)". The Physics Factbook.
- http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000
- Geologic Time Scale – as 18 galactic rotations
- Williams, Caroline; Nield, Ted (20 October 2007). "Pangaea, the comeback". New Scientist. Retrieved 2 January 2014.
- Franck, S.; Bounama, C.; Von Bloh, W. (November 2005). "Causes and timing of future biosphere extinction" (PDF). Biogeosciences Discussions 2 (6): 1665–1679. Bibcode:2005BGD.....2.1665F. doi:10.5194/bgd-2-1665-2005. Retrieved 19 October 2011.
- Luhmann, J. G.; Johnson, R. E.; Zhang, M. H. G. (1992). "Evolutionary impact of sputtering of the Martian atmosphere by O+ pickup ions". Geophysical Research Letters 19 (21): 2151–2154. Bibcode:1992GeoRL..19.2151L. doi:10.1029/92GL02485.
- Quirin Shlermeler (3 March 2005). "Solar wind hammers the ozone layer". nature news. doi:10.1038/news050228-12.
- Cox, J. T.; Loeb, Abraham (2007). "The Collision Between The Milky Way And Andromeda". Monthly Notices of the Royal Astronomical Society 386 (1): 461. arXiv:0705.1170. Bibcode:2008MNRAS.tmp..333C. doi:10.1111/j.1365-2966.2008.13048.x.
- Loeb, Abraham (2011). "Cosmology with Hypervelocity Stars". Harvard University. arXiv:1102.0007v2.
- "The Local Group of Galaxies". University of Arizona. Students for the Exploration and Development of Space. Retrieved 2 October 2009.
- Adams, Fred C.; Laughlin, Gregory (April 1997). "A dying universe: the long-term fate and evolution of astrophysical objects". Reviews of Modern Physics 69 (2): 337–372. arXiv:astro-ph/9701131. Bibcode:1997RvMP...69..337A. doi:10.1103/RevModPhys.69.337.
- "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 neighbourhood of the Sun.