Galactic year

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

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 is 230 million 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.

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

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
6.8 galactic years agoMulticellular organisms appear
6.666 galactic years agoEukaryotes appear
2.4 galactic years agoCambrian explosion occurs
2 galactic years agoThe first brain structure appears in worms
1.11 galactic year agoPermian–Triassic extinction event
0.2935 galactic years agoCretaceous–Paleogene extinction event
Present day
0.15 galactic year from nowMean time between impacts of asteroidal bodies in the order of magnitude of the K/Pg impactor has elapsed.[5]
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.[6]
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[7]
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 complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda[8]
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 [9]
2000 galactic years from nowLocal Group of 47 galaxies[10] coalesces into a single large galaxy [11]
Visualisation of the orbit of the Sun (yellow dot and white curve) around the Galactic Centre (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 programme.[12]
Visualisation of the orbit of the Sun (yellow dot and white curve) around the Galactic Centre (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 programme.[12]

See also

  • Galactic Tick Day


  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. ^ NASA – StarChild Question of the Month for February 2000
  4. ^ Geologic Time Scale – as 18 galactic rotations
  5. ^ Lunar and Planetary Institute (2010),
  6. ^ Williams, Caroline; Nield, Ted (20 October 2007). "Pangaea, the comeback". New Scientist. Retrieved 2 January 2014.
  7. ^ Franck, S.; Bounama, C.; von Bloh, W. (2005-11-07). "Causes and timing of future biosphere extinction". Biogeosciences Discussions. Copernicus GmbH. 2 (6): 1665-1679. Bibcode:2005BGD.....2.1665F. doi:10.5194/bgd-2-1665-2005.
  8. ^ Cox, T. J.; Loeb, Abraham (2008-05-01). "The collision between the Milky Way and Andromeda". Monthly Notices of the Royal Astronomical Society. Oxford University Press (OUP). 386 (1): 461–474. arXiv:0705.1170. Bibcode:2008MNRAS.tmp..333C. doi:10.1111/j.1365-2966.2008.13048.x. ISSN 0035-8711. S2CID 14964036.
  9. ^ Loeb, Abraham (2011-04-18). "Cosmology with hypervelocity stars". Journal of Cosmology and Astroparticle Physics. IOP Publishing. 2011 (4): 023. arXiv:1102.0007. Bibcode:2011JCAP...04..023L. doi:10.1088/1475-7516/2011/04/023. ISSN 1475-7516. S2CID 118750775.
  10. ^ "The Local Group of Galaxies". University of Arizona. Students for the Exploration and Development of Space. Retrieved 2 October 2009.
  11. ^ 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.
  12. ^ "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.

Media files used on this page

Author/Creator: ESO, European Southern Observatory, Licence: CC BY 4.0
Artist's impression of "the oldest star of our Galaxy": HE 1523-0901
  • About 13.2 billion years old
  • Approximately 7500 light years far from Earth
  • Published as part of Hamburg/ESO Survey in the May 10 2007 issue of The Astrophysical Journal
Author/Creator: unknown, Licence: CC BY 2.5
WP:Timeline Tracer logo
Crab Nebula.jpg
This is a mosaic image, one of the largest ever taken by NASA's Hubble Space Telescope, of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event in 1054 CE, as did, almost certainly, Native Americans.

The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second due to the neutron star's rotation. A neutron star is the crushed ultra-dense core of the exploded star.

The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844, using a 36-inch telescope. When viewed by Hubble, as well as by large ground-based telescopes such as the European Southern Observatory's Very Large Telescope, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of a star, 6,500 light-years away.

The newly composed image was assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.
Author/Creator: Me, Licence: Copyrighted free use
SVG replacement for File:Spaceship and the Sun.jpg. A stylized illustration of a spaceship and the sun, based on the description of the emblem of the fictional Galactic Empire in Isaac Asimov's Foundation series ("The golden globe with its conventionalized rays, and the oblique cigar shape that was a space vessel"). This image could be used as a icon for science-fiction related articles.
This view of the rising Earth greeted the Apollo 8 astronauts as they came from behind the Moon after the fourth nearside orbit. Earth is about five degrees above the horizon in the photo. The unnamed surface features in the foreground are near the eastern limb of the Moon as viewed from Earth. The lunar horizon is approximately 780 kilometers from the spacecraft. Width of the photographed area at the horizon is about 175 kilometers. On the Earth 240,000 miles away, the sunset terminator bisects Africa.
Solar system.jpg
This is a montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune. The spacecraft responsible for these images are as follows:
  • the Mercury image was taken by Mariner 10,
  • the Venus image by Magellan,
  • the Earth and Moon images by Galileo,
  • the Mars image by Mars Global Surveyor,
  • the Jupiter image by Cassini, and
  • the Saturn, Uranus and Neptune images by Voyager.
  • Pluto is not shown as it is no longer a planet, and no spacecraft has yet visited it when this montage was taken. The inner planets (Mercury, Venus, Earth, Moon, and Mars) are roughly to scale to each other; the outer planets (Jupiter, Saturn, Uranus, and Neptune) are roughly to scale to each other. PIA 00545 is the same montage with Neptune shown larger in the foreground. Actual diameters are given below:
  • Sun (to photosphere) 1,392,684 km
  • Mercury 4,879.4 km
  • Venus 12,103.7 km
  • Earth 12,756.28 km
  • Moon 3,476.2 km
  • Mars 6,804.9 km
  • Jupiter 142,984 km
  • Saturn 120,536 km
  • Uranus 51,118 km
  • Neptune 49,528 km
Sun in orbit around Galactic Centre.gif
Author/Creator: ESO, European Southern Observatory, Licence: CC BY 4.0
Stars studied during a 15-year monitoring programme making their most recent orbital revolution around the Galactic centre before converging into the small volume where they were observed by the team. The duration of the video corresponds to about 250 million years. The yellow dot and white curve show how the Sun moved during this last of its about 20 laps around our Galaxy.
Milky Way Arms ssc2008-10.svg
Artist's conception of the Milky Way galaxy as seen from far Galactic North (in Coma Berenices) by NASA/JPL-Caltech/R. Hurt [1] annotated with arms (colour-coded according to Milky Way article) as well as distances from the Solar System and galactic longitude with corresponding constellation.