Galactic year

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

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. One galactic year is approximately 230 million Earth years. 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, 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.

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

The following list assumes that 1 galactic year is 225 million years.

Time		Event
Galactic years (gal)	Millions of years (Ma)	Event
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 first appearance of eukaryotes 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.
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.
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
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
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
2000 gal		Local Group of 47 galaxies coalesces into a single large galaxy

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.

Time measurement and standards
Chronometry Orders of magnitude Metrology
International standards	Coordinated Universal Time offset UT ΔT DUT1 International Earth Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line IERS Reference Meridian Leap second Solar time Terrestrial Time Time zone 180th meridian
Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian
Time in physics	Absolute space and time Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain Time translation symmetry T-symmetry
Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch stopwatch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema
Calendar	Gregorian Hebrew Hindu Holocene Islamic (lunar Hijri) Julian Solar Hijri Astronomical Dominical letter Epact Equinox Intercalation Julian date Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names
Archaeology and geology	Chronological dating Geologic time scale International Commission on Stratigraphy
Astronomical chronology	Galactic year Nuclear timescale Precession Sidereal time
Other units of time	Instant Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium
Related topics	Chronology Duration music Mental chronometry Decimal time Metric time System time Time metrology Time value of money Timekeeper

Galactic year

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

See also