View source for Astronomy:Galactic year

{{short description|Unit of time}}
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The '''galactic year''', also known as a '''cosmic year''', is the duration of time required for the [[Astronomy:Sun|Sun]] to [[Physics:Orbit|orbit]] once around the [[Galactic Center|center]] of the Milky Way Galaxy.<ref name="akb">[http://www.csi.uottawa.ca:4321/astronomy/index.html#cosmicyear Cosmic Year] {{webarchive|url=https://web.archive.org/web/20140412221436/http://www.csi.uottawa.ca:4321/astronomy/index.html |date=2014-04-12 }}, Fact Guru, University of Ottawa</ref> One galactic year is approximately 225 million Earth years.<ref name="biblio">{{cite web|url=http://hypertextbook.com/facts/2002/StacyLeong.shtml|work=The Physics Factbook|last=Leong|first=Stacy|year=2002|title=Period of the Sun's Orbit around the Galaxy (Cosmic Year)}}</ref>  The Solar System is traveling at an average speed of 230&nbsp;km/s (828,000&nbsp;km/h) or 143&nbsp;mi/s (514,000&nbsp;mph) within its trajectory around the Galactic Center,<ref name="StarChild">http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000</ref> 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.<ref name="timeline">[http://www.vendian.org/mncharity/dir3/geologic_time_galactic Geologic Time Scale – as 18 galactic rotations]</ref>

==Timeline of the universe and Earth's history in galactic years==
{{main|Astronomy:Timeline of the early universe|Earth:Timeline of natural history|Unsolved:Timeline of the far future}}
[[File:Motion of Sun, Earth and Moon around the Milky Way.jpg|thumb|The orientation of the Solar System's motion]]
The following list assumes that 1 galactic year is 225 million years.

{| class="wikitable"
! colspan=2|Time
! rowspan=2|Event
|-
!Galactic<br>years<br>(gal)
!Millions<br>of years<br>(Ma)
|-
! colspan="3" style="background:#EEEEFF;"|Past (years ago)
|-
|align="right"|About 61.32 gal
| ||[[Astronomy:Big Bang|Big Bang]]
|-
|align="right"|About 54 gal
| ||Birth of the [[Astronomy:Milky Way|Milky Way]]
|-
|align="right"|20.44 gal
| ||Birth of the [[Astronomy:Sun|Sun]]
|-
|align="right"|17–18 gal
| 3937 Ma||Oceans appear on Earth
|-
|align="right"|16.889 gal
| 3800 Ma||[[Biology:Timeline of the evolutionary history of life|Life begins on Earth]]
|-
|align="right"|15.555 gal
| 3500 Ma||Prokaryotes appear
|-
|align="right"|12 gal
| 2700 Ma||[[Biology:Bacteria|Bacteria]] appear
|-
|align="right"|10 gal
|-
| align="right" |6.8 gal
| 1530 Ma||Multicellular organisms appear
|-
| align="right" |2.4 gal
| 540 Ma||[[Earth:Cambrian explosion|Cambrian explosion]] occurs
|-
| align="right" |2 gal
| 500 Ma||The first brain structure appears in worms
|-
| align="right" |1.11 gal
| 250 Ma||[[Earth:Permian–Triassic extinction event|Permian–Triassic extinction event]]
|-
| align="right" |0.2933 gal
| ||[[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]]
|-
| align="right" |0.0013 gal
| ||Emergence of anatomically modern humans
|-
! colspan="3" style="background:#EEEEFF;"|Future (years from now)
|-
| align="right" |0.15 gal
|
|Mean time between impacts of asteroidal bodies in the order of magnitude of the K/Pg impactor has elapsed.<ref>Lunar and Planetary Institute (2010), https://www.lpi.usra.edu/features/chicxulub/</ref>
|-
| align="right" |1 gal
|
|All the continents on Earth may fuse into a [[Earth:Supercontinent|supercontinent]]. Three potential arrangements of this configuration have been dubbed [[Earth:Amasia (continent)|Amasia]], [[Earth:Novopangaea|Novopangaea]], and [[Earth:Pangaea Ultima|Pangaea Ultima]].<ref>{{cite web |last1=Williams |first1=Caroline  |last2=Nield |first2=Ted |date=2007-10-17 |title=Pangaea, the comeback |work=New Scientist |accessdate=2014-01-02 |url=https://www.newscientist.com/article/mg19626261-500-pangaea-the-comeback/ }}</ref>
|-
| align="right" |2–3 gal
|
|[[Astronomy:Tidal acceleration|Tidal acceleration]] moves the Moon far enough from Earth that total solar eclipses are no longer possible
|-
| align="right" |4 gal
|
|Carbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out<ref>{{cite journal| last1=Franck | first1=S. | last2=Bounama | first2=C. | last3=von Bloh | first3=W. | title=Causes and timing of future biosphere extinction |journal=Biogeosciences Discussions| publisher=Copernicus GmbH |volume=2|issue=6| date=2005-11-07 | doi=10.5194/bgd-2-1665-2005 | page=1665-1679|bibcode=2006BGeo....3...85F| s2cid=3619702 | doi-access=free }}</ref>
|-
| align="right" |15 gal
|
|Surface conditions on [[Astronomy:Earth|Earth]] are comparable to those on [[Astronomy:Venus|Venus]] today
|-
| align="right" |22 gal
| ||The Milky Way and [[Astronomy:Andromeda Galaxy|Andromeda Galaxy]] [[Astronomy:Andromeda–Milky Way collision|begin to collide]]
|-
| align="right" |25 gal
| ||Sun ejects a [[Astronomy:Planetary nebula|planetary nebula]], leaving behind a [[Astronomy:White dwarf|white dwarf]]
|-
| align="right" |30 gal
| ||The Milky Way and Andromeda complete their merger into a giant [[Astronomy:Elliptical galaxy|elliptical galaxy]] called Milkomeda or Milkdromeda<ref>{{cite journal | last1=Cox | first1=T. J. | last2=Loeb | first2=Abraham | title=The collision between the Milky Way and Andromeda | journal=Monthly Notices of the Royal Astronomical Society | publisher=Oxford University Press (OUP) | volume=386 | issue=1 | date=2008-05-01 | issn=0035-8711 | doi=10.1111/j.1365-2966.2008.13048.x | pages=461–474|bibcode=2008MNRAS.386..461C|arxiv=0705.1170| s2cid=14964036 }}</ref>
|-
| align="right" |500 gal
| ||The Universe's expansion causes all galaxies beyond the Milky Way's [[Astronomy:Local Group|Local Group]] to disappear beyond the cosmic light horizon, removing them from the observable universe<ref>{{cite journal | last=Loeb | first=Abraham | title=Cosmology with hypervelocity stars | journal=Journal of Cosmology and Astroparticle Physics | publisher=IOP Publishing | volume=2011 | issue=4 | date=2011-04-18 | issn=1475-7516 | doi=10.1088/1475-7516/2011/04/023 | pages=023|arxiv=1102.0007| bibcode=2011JCAP...04..023L | s2cid=118750775 }}</ref>
|-
| align="right" |2000 gal
| ||Local Group of 47 galaxies<ref>{{cite web |title=The Local Group of Galaxies |work=University of Arizona |first1=Hartmut |last1=Frommert |first2=Christine |last2=Kronberg |publisher=Students for the Exploration and Development of Space |date=2007-06-05 |accessdate=2009-10-02 |url=http://www.seds.org/messier/more/local.html |archiveurl=https://web.archive.org/web/20090907170724/http://www.seds.org/messier/more/local.html |archivedate=2009-09-07 }}</ref> coalesces into a single large galaxy<ref>{{cite journal | last1=Adams | first1=Fred C. | last2=Laughlin | first2=Gregory | title=A dying universe: the long-term fate and evolutionof astrophysical objects | journal=Reviews of Modern Physics | volume=69 | issue=2 | date=1997-04-01 | issn=0034-6861 | doi=10.1103/revmodphys.69.337 | pages=337–372|bibcode=1997RvMP...69..337A|arxiv=astro-ph/9701131| s2cid=12173790 }}</ref>
|}
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 |oLeft = 140
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 |Location = center
 |Description = 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 [[Organization:European Southern Observatory|European Southern Observatory]] in a monitoring programme.<ref>{{cite web|title=Milky Way Past Was More Turbulent Than Previously Known | url= http://www.eso.org/public/news/eso0411/ |work=ESO News|publisher = [[Organization:European Southern Observatory|European Southern Observatory]] |date = 2004-04-06 |quote= 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.}}</ref>
 }}

== See also ==

* [[Astronomy:Galactic Tick Day|Galactic Tick Day]]
* [[Earth:Geologic time scale|Geologic time scale]]

==References==
{{Reflist}}

{{Star}}
{{Time measurement and standards}}
{{Chronology}}
{{DEFAULTSORT:Galactic Year}}
[[Category:Milky Way]]

{{Sourceattribution|Galactic year}}