Earth's location in the Universe

Knowledge of Earth's location in the Universe has been shaped by 400 years of telescopic observations, and has expanded radically in the last century. Initially, Earth was believed to be the center of the Universe, which consisted only of those planets visible with the naked eye and an outlying sphere of fixed stars.^[1] After the acceptance of the heliocentric model in the 17th century, observations by William Herschel and others showed that the Sun lay within a vast, disc-shaped galaxy of stars.^[2] By the 20th century, observations of spiral nebulae by Edwin Hubble revealed that our galaxy was one of billions in an expanding universe,^[3][4] grouped into clusters and superclusters. By the end of the 20th century, the overall structure of the visible universe was becoming clearer, with superclusters forming into a vast web of filaments and voids.^[5] Superclusters, filaments and voids are the largest coherent structures in the Universe that we can observe.^[6] At still larger scales (over 1000 megaparsecs^{[lower-alpha 1]}) the Universe becomes homogeneous meaning that all its parts have on average the same density, composition and structure.^[7]

Since there is believed to be no "center" or "edge" of the Universe, there is no particular reference point with which to plot the overall location of the Earth in the universe.^[8] Because the observable universe is defined as that region of the Universe visible to terrestrial observers, Earth is, by definition, the center of the observable universe.^[9] Reference can be made to the Earth's position with respect to specific structures, which exist at various scales. It is still undetermined whether the Universe is infinite. There have been numerous hypotheses that our universe may be only one such example within a higher multiverse; however, no direct evidence of any sort of multiverse has ever been observed, and some have argued that the hypothesis is not falsifiable.^[10][11]

A diagram of our location in the observable universe.

Feature	Diameter	Notes	Sources
Earth	12,756.2 km (equatorial)	Measurement comprises just the solid part of the Earth; there is no agreed upper boundary for Earth's atmosphere. The geocorona, a layer of UV-luminescent hydrogen atoms, lies at 100,000 km. The Kármán line, defined as the boundary of space for astronautics, lies at 100 km.	[12][13][14][15]
Orbit of the Moon	768,210 km[lower-alpha 2]	The average diameter of the orbit of the Moon relative to the Earth.	[16]
Geospace	6,363,000– 12,663,000 km (110-210 Earth radii)	The space dominated by Earth's magnetic field and its magnetotail, shaped by the solar wind.	[17]
Earth's orbit	299.2 million km[lower-alpha 2] 2 AU[lower-alpha 3]	The average diameter of the orbit of the Earth relative to the Sun. Encompasses the Sun, Mercury and Venus.	[18]
Inner Solar System	~6.54 AU	Encompasses the Sun, the inner planets (Mercury, Venus, Earth, Mars) and the asteroid belt. Cited distance is the 2:1 resonance with Jupiter, which marks the outer limit of the asteroid belt.	[19][20][21]
Outer Solar System	60.14 AU	Includes the outer planets (Jupiter, Saturn, Uranus, Neptune). Cited distance is the orbital diameter of Neptune.	[22]
Kuiper belt	~96 AU	Belt of icy objects surrounding the outer Solar System. Encompasses the dwarf planets Pluto, Haumea and Makemake. Cited distance is the 2:1 resonance with Neptune, generally regarded as the outer edge of the main Kuiper belt.	[23]
Heliosphere	160 AU	Maximum extent of the solar wind and the interplanetary medium.	[24][25]
Scattered disc	195.3 AU	Region of sparsely scattered icy objects surrounding the Kuiper belt. Encompasses the dwarf planet Eris. Cited distance is derived by doubling the aphelion of Eris, the farthest known scattered disc object. As of now, Eris's aphelion marks the farthest known point in the scattered disc.	[26]
Oort cloud	100,000–200,000 AU 0.613–1.23 pc[lower-alpha 1]	Spherical shell of over a trillion (10^12) comets. Existence is currently hypothetical, but inferred from the orbits of long-period comets.	[27]
Solar System	1.23 pc	The Sun and its planetary system. Cited diameter is that of the Sun's Hill sphere; the region of its gravitational influence.	[28]
Local Interstellar Cloud	9.2 pc	Interstellar cloud of gas through which the Sun and a number of other stars are currently travelling.	[29]
Local Bubble	2.82–250 pc	Cavity in the interstellar medium in which the Sun and a number of other stars are currently travelling. Caused by a past supernova.	[30][31]
Gould Belt	1,000 pc	Ring of young stars through which the Sun is currently travelling.	[32]
Orion Arm	3000 pc (length)	The spiral arm of the Milky Way Galaxy through which the Sun is currently travelling.	[33]
Orbit of the Solar System	17,200 pc	The average diameter of the orbit of the Solar System relative to the Galactic Center. The Sun's orbital radius is roughly 8,600 parsecs, or slightly over half way to the galactic edge. One orbital period of the Solar System lasts between 225 and 250 million years.	[34][35]
Milky Way Galaxy	30,000 pc	Our home galaxy, composed of 200 billion to 400 billion stars and filled with the interstellar medium.	[36][37]
Milky Way subgroup	840,500 pc	The Milky Way and those satellite dwarf galaxies gravitationally bound to it. Examples include the Sagittarius Dwarf, the Ursa Minor Dwarf and the Canis Major Dwarf. Cited distance is the orbital diameter of the Leo T Dwarf galaxy, the most distant galaxy in the Milky Way subgroup.	[38]
Local Group	3 Mpc[lower-alpha 1]	Group of at least 47 galaxies of which the Milky Way is a part. Dominated by Andromeda (the largest), the Milky Way and Triangulum; the remainder are dwarf galaxies.	[39]
Local Sheet	7 Mpc	Group of galaxies including the Local Group moving at the same relative velocity towards the Virgo Cluster and away from the Local Void.	[40][41]
Virgo Supercluster	30 Mpc	The supercluster of which the Local Group is a part. It comprises roughly 100 galaxy groups and clusters, centred on the Virgo Cluster. The Local Group is located on the outer edge of the Virgo Supercluster.	[42][43]
Laniakea	160 Mpc	A group connected with the superclusters of which the Local Group is a part. Comprises roughly 300 to 500 galaxy groups and clusters, centred on the Great Attractor in the Hydra-Centaurus Supercluster.	[44][45][46][47]
Observable universe	28,000 Mpc	More than 100 billion galaxies, arranged in millions of superclusters, galactic filaments, and voids, creating a foam-like superstructure.	[48][49]
Universe	Minimum 28,000 Mpc	Beyond the observable universe lie the unobservable regions from which no light has reached the Earth yet. No information is available, as light is the fastest travelling medium of information. However, uniformitarianism argues that the Universe is likely to contain more galaxies in the same foam-like superstructure.	[50]

See also

• Orders of magnitude (length)
• Cosmic View
• Cosmic Zoom
• Pale Blue Dot
• Powers of Ten (film)
• Galaxy Song

Notes

1. A parsec (pc) is the distance at which a star's parallax as viewed from Earth is equal to one second of arc, equal to roughly 206,000 AU or 3.0857×10¹³ km. One megaparsec (Mpc) is equivalent to one million parsecs.
2. Semi-major and semi-minor axes.
3. 1 AU or astronomical unit is the distance between the Earth and the Sun, or 150 million km. Earth's orbital diameter is twice its orbital radius, or 2 AU.

References

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