வியாழன், 31 ஆகஸ்ட், 2017

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 agoBig Bang
About 54 galactic years agoBirth of the Milky Way
20.44 galactic years agoBirth of the Sun
17–18 galactic years agoOceans appear on Earth
16.889 galactic years agoLife begins on Earth
15.555 galactic years agoProkaryotes appear
12 galactic years agoBacteria appear
10 galactic years agoStable continents appear
6.666 galactic years agoEukaryotes appear
6.8 galactic years agoMulticellular organisms appear
2.4 galactic years agoCambrian explosion occurs
2 galactic years agoThe first brain structure appeared in worms
1.11 galactic year agoPermian–Triassic extinction event
0.2935 galactic years agoCretaceous–Paleogene extinction event
0.001 galactic years agoAppearance of modern humans
Present day
1 galactic year from nowAll 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 nowTidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible
4 galactic years from nowCarbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out[6]
12 galactic years from nowThe Earth's magnetic field shuts down [7] and charged particles emanating from the Sun gradually deplete the atmosphere [8]
15 galactic years from nowSurface conditions on Earth are comparable to those on Venus today
22 galactic years from nowThe Milky Way and Andromeda Galaxy begin to collide
25 galactic years from nowSun ejects a planetary nebula, leaving behind a white dwarf
30 galactic years from nowThe Milky Way and Andromeda will complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda [9]
500 galactic years from nowThe 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 nowMedian Point of the Local Group of 47 galaxies[11] will coalesce into a single large galaxy 

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