Galactic year

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 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]

Milky Way Arms ssc2008-10
Approximate orbit of the Sun (yellow circle) around the Galactic Centre

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 ago Big Bang
About 54 galactic years ago Birth of the Milky Way
20.44 galactic years ago Birth of the Sun
17–18 galactic years ago Oceans appear on Earth
16.889 galactic years ago Life begins on Earth
15.555 galactic years ago Prokaryotes appear
12 galactic years ago Bacteria appear
10 galactic years ago Stable continents appear
6.8 galactic years ago Multicellular organisms appear
6.666 galactic years ago Eukaryotes appear
2.4 galactic years ago Cambrian explosion occurs
2 galactic years ago The first brain structure appears in worms
1.11 galactic year ago Permian–Triassic extinction event
0.2935 galactic years ago Cretaceous–Paleogene extinction event
Present day
1 galactic year from now All 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 now Tidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible
4 galactic years from now Carbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out[6]
12 galactic years from now The Earth's magnetic field shuts down [7] and charged particles emanating from the Sun gradually deplete the atmosphere [8]
15 galactic years from now Surface conditions on Earth are comparable to those on Venus today
22 galactic years from now The Milky Way and Andromeda Galaxy begin to collide
25 galactic years from now Sun ejects a planetary nebula, leaving behind a white dwarf
30 galactic years from now The Milky Way and Andromeda complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda [9]
500 galactic years from now 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 [10]
2000 galactic years from now Local Group of 47 galaxies[11] coalesces into a single large galaxy [12]
Sun in orbit around Galactic Centre
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.[13]


  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. ^ Williams, Caroline; Nield, Ted (20 October 2007). "Pangaea, the comeback". New Scientist. Retrieved 2 January 2014.
  6. ^ Franck, S.; Bounama, C.; Von Bloh, W. (November 2005). "Causes and timing of future biosphere extinction" (PDF). Biogeosciences Discussions 2 (6): 1665–1679. Bibcode2005BGD.....2.1665F. doi:10.5194/bgd-2-1665-2005. Retrieved 19 October 2011.
  7. ^ 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. Bibcode1992GeoRL..19.2151L. doi:10.1029/92GL02485.
  8. ^ Quirin Shlermeler (3 March 2005). "Solar wind hammers the ozone layer". nature news. doi:10.1038/news050228-12.
  9. ^ 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:arXiv:0705.1170. Bibcode2008MNRAS.tmp..333C. doi:10.1111/j.1365-2966.2008.13048.x.
  10. ^ Loeb, Abraham (2011). "Cosmology with Hypervelocity Stars". Harvard University.arXiv:1102.0007.
  11. ^ "The Local Group of Galaxies". University of Arizona. Students for the Exploration and Development of Space. Retrieved 2 October 2009.
  12. ^ 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. Bibcode1997RvMP...69..337A. doi:10.1103/RevModPhys.69.337.
  13. ^ "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.

Circa (from Latin, meaning 'around, about') – frequently abbreviated c., ca. or ca and less frequently circ. or cca. – signifies "approximately" in several European languages and as a loanword in English, usually in reference to a date. Circa is widely used in historical writing when the dates of events are not accurately known.

When used in date ranges, circa is applied before each approximate date, while dates without circa immediately preceding them are generally assumed to be known with certainty.


1732–1799: Both years are known precisely.

c. 1732 – 1799: The beginning year is approximate; the end year is known precisely.

1732 – c. 1799: The beginning year is known precisely ; the end year is approximate.

c. 1732 – c. 1799: Both years are approximate.

Era (geology)

A geologic era is a subdivision of geologic time that divides an eon into smaller units of time. The Phanerozoic Eon is divided into three such time frames: the Paleozoic, Mesozoic, and Cenozoic (meaning "old life", "middle life" and "recent life") that represent the major stages in the macroscopic fossil record. These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-Pg boundary between the Mesozoic and the Cenozoic. There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.

The Hadean, Archean and Proterozoic eons were as a whole formerly called the Precambrian. This covered the four billion years of Earth history prior to the appearance of hard-shelled animals. More recently, however, the Archean and Proterozoic eons have been subdivided into eras of their own.

Geologic eras are further subdivided into geologic periods, although the Archean eras have yet to be subdivided in this way.


Floruit (UK: , US: ), abbreviated fl. (or occasionally flor.), Latin for "he/she flourished", denotes a date or period during which a person was known to have been alive or active. In English, the word may also be used as a noun indicating the time when someone flourished.

Frozen star (hypothetical star)

In astronomy, a frozen star, besides a disused term for a black hole, is a type of hypothetical star that, according to the astronomers Fred Adams and Gregory P. Laughlin, may appear in the future of the Universe when the metallicity of the interstellar medium is several times the solar value.

Geological period

A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place.

These periods form elements of a hierarchy of divisions into which geologists have split the Earth's history.

Eons and eras are larger subdivisions than periods while periods themselves may be divided into epochs and ages.

The rocks formed during a period belong to a stratigraphic unit called a system.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Star Wars Tales Volume 1

Star Wars Tales Volume 1 is the first Star Wars Tales trade paperback, collecting issues 1-4.

Star Wars Tales Volume 2

Star Wars Tales Volume 2 is the second Star Wars Tales trade paperback, collecting issues 5-8.

Star Wars Tales Volume 3

Star Wars Tales Volume 3 is the third Star Wars Tales trade paperback, collecting issues 9-12.

Star Wars Tales Volume 4

Star Wars Tales Volume 4 is the fourth Star Wars Tales trade paperback, collecting issues 13-16.

Star Wars Tales Volume 5

Star Wars Tales Volume 5 is the fifth Star Wars Tales trade paperback, collecting issues 17-20.

Star Wars Tales Volume 6

Star Wars Tales Volume 6 is the sixth Star Wars Tales trade paperback, collecting issues 21-24.

Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Stellar mass

Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (M☉). Hence, the bright star Sirius has around 2.02 M☉. A star's mass will vary over its lifetime as additional mass becomes accreted, such as from a companion star, or mass is ejected with the stellar wind or pulsational behavior.

Unit of time

A unit of time or midst unit is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern definition, from the National Institute of Standards and Technology is:

The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.Historically units of time were defined by the movements of astronomical objects.

Sun based: the year was the time for the earth to rotate around the sun. Year-based units include the olympiad (four years), the lustrum (five years), the indiction (15 years), the decade, the century, and the millennium.

Moon based: the month was based on the moon's orbital period around the earth.

Earth based: the time it took for the earth to rotate on its own axis, as observed on a sundial. Units originally derived from this base include the week at seven days, and the fortnight at 14 days. Subdivisions of the day include the hour (1/24th of a day) which was further subdivided into minutes and finally seconds. The second became the international standard unit (SI units) for science.

Celestial sphere based: as in sidereal time, where the apparent movement of the stars and constellations across the sky is used to calculate the length of a year.These units do not have a consistent relationship with each other and require intercalation. For example, the year cannot be divided into 12 28-day months since 12 times 28 is 336, well short of 365. The lunar month (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the Gregorian calendar as 365.24 days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined as multiples of seconds.

Units of time based on orders of magnitude of the second include the nanosecond and the millisecond.


A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility. The current year is 2019.

In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn, and winter. In tropical and subtropical regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.

A calendar year is an approximation of the number of days of the Earth's orbital period as counted in a given calendar. The Gregorian calendar, or modern calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars; see below. For the Gregorian calendar, the average length of the calendar year (the mean year) across the complete leap cycle of 400 years is 365.2425 days. The ISO standard ISO 80000-3, Annex C, supports the symbol a (for Latin annus) to represent a year of either 365 or 366 days. In English, the abbreviations y and yr are commonly used.

In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.The word year is also used for periods loosely associated with, but not identical to, the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. Similarly, year can mean the orbital period of any planet; for example, a Martian year and a Venusian year are examples of the time a planet takes to transit one complete orbit. The term can also be used in reference to any long period or cycle, such as the Great Year.

Luminosity class
Hypothetical stars
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Time in physics
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