To help compare different orders of magnitude, the following list describes various lengths between $1.6\times 10^{-35}$ metres and $10^{10^{10^{122}}}$metres.
Since 1983, defined as length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. See History of the metre for previous definitions.
2.72 m
Height of Robert Wadlow, tallest known human being.^{[27]}
Lower bound of the (possibly infinite) radius of the universe, if it is a 3-sphere, according to one estimate using the WMAP data at 95% confidence^{[44]} It equivalently implies that there are at minimum 21 particle horizon-sized volumes in the universe.
$10^{10^{115}}$^{[note 3]}
$10^{10^{115}}$ Ym
$10^{10^{115}}$Ym
According to the laws of probability, the distance one must travel until one encounters a volume of space identical to our observable universe with conditions identical to our own.^{[45]}^{[46]}
$10^{10^{10^{122}}}$
$10^{10^{10^{122}}}$ Ym
$10^{10^{10^{122}}}$Ym
Maximum size of universe after cosmological inflation, implied by one resolution of the No-Boundary Proposal^{[47]}
1.6 × 10^{−11}yoctometres (1.6 × 10^{−35} metres) – the Planck length (Measures of distance shorter than this are considered nonsensical and do not make any physical sense, according to current theories of physics).
1 ym – 1 yoctometre, the smallest named subdivision of the metre in the SIbase unit of length, one septillionth of a metre
To help compare different orders of magnitude, this section lists lengths between 10^{−23}metres and 10^{−22} metres (10 ym and 100 ym).
100 yoctometres
To help compare different orders of magnitude, this section lists lengths between 10^{−22}m and 10^{−21} m (100 ym and 1 zm).
100 ym – length of a top quark, one of the smallest known quarks
1 zeptometre
To help compare different orders of magnitude, this section lists lengths between 10^{−21}m and 10^{−20} m (1 zm and 10 zm).
2 zm – length of a preon, hypothetical particles proposed as subcomponents of quarks and leptons; the upper bound for the width of a cosmic string in string theory.
To help compare different orders of magnitude, this section lists lengths between 10^{−13}m and 10^{−12} m (100 fm and 1 pm).
570 fm – typical distance from the atomic nucleus of the two innermost electrons (electrons in the 1s shell) in the uranium atom, the heaviest naturally-occurring atom
1 picometre
To help compare different orders of magnitude this section lists lengths between 10^{−12} and 10^{−11}m (1 pm and 10 pm).
Comparison of sizes of semiconductor manufacturing process nodes with some microscopic objects and visible light wavelengths. At this scale, the width of a human hair is about 10 times that of the image.^{[55]}
300 nm – greatest particle size that can fit through a HEPA (High Efficiency Particulate Air) filter (N100 removes up to 99.97% at 0.3 micrometres, N95 removes up to 95% at 0.3 micrometres)
To help compare different orders of magnitude, this section lists some items with lengths between 10^{−6} and 10^{−5}m (between 1 and 10 micrometres, or μm).
To help compare different orders of magnitude, this section lists lengths between 10^{−4}m and 10^{−3} m (100 μm and 1 mm). The term myriometre (abbr. mom, equivalent to 100 micrometres; frequently confused with the myriametre, 10 kilometres)^{[68]} is deprecated; the decimal metric prefix myrio-^{[69]} is obsolete^{[70]}^{[71]}^{[72]} and was not included among the prefixes when the International System of Units was introduced in 1960.
100 μm – 1/10 of a millimetre
100 μm – 0.00394 inches
100 μm – smallest distance that can be seen with the naked eye
100 μm – average diameter of a strand of human hair^{[21]}
66 cm = 6.6 dm – length of the longest pine cones (produced by the sugar pine^{[84]})
Astronomical
84 cm = 8.4 dm – approximate diameter of 2008 TS26, a meteoroid
1 metre
Leonardo da Vinci drew the Vitruvian Man within a square of side 1.83 metres and a circle about 1.2 metres in radius
To help compare different orders of magnitude, this section lists lengths between one metre and ten metres.
Light travels 1 metre in ^{1}⁄_{299,792,458}, or 3.3356409519815E-9 of a second.
15 metres – approximate distance the tropical circles of latitude are moving towards the equator and the polar circles are moving towards the poles each year due to a natural, gradual decrease in the Earth's axial tilt
To help compare different orders of magnitude, this section lists lengths between 10 and 100 kilometres (10^{4} to 10^{5}metres). The myriametre^{[111]} (sometimes also spelled myriameter, myriometre and myriometer) (10,000 metres) is a deprecated unit name; the decimal metric prefix myria-^{[69]} (sometimes also written as myrio-^{[112]}^{[113]}^{[114]}) is obsolete^{[70]}^{[71]}^{[72]} and not included among the prefixes when the International System of Units was introduced in 1960.
Conversions
10 kilometres is equal to:
Distance marker on the Rhine: 36 (XXXVI) myriametres from Basel. The stated distance is 360 km; comma is the decimal separator in Germany.
A length of 100 kilometres (about 62 miles), as a rough amount, is relatively common in measurements on Earth and for some astronomical objects.
It is the altitude at which the FAI defines spaceflight to begin.
472 km – diameter of Miranda, one of Uranus' major moons
974.6 km – greatest diameter of 1 Ceres,^{[31]} the largest solar system asteroid^{[note 2]}
1 megametre
Small planets, the Moon and dwarf planets in our solar system have diameters from one to ten million metres. Top row: Mars (left), Mercury (right); bottom row: Moon (left), Pluto (center), and Haumea (right), to scale.
Planets from Venus up to Uranus have diameters from ten to one hundred million metres. Top row: Uranus (left), Neptune (right); middle row: Earth (left), Sirius B (center), and Venus (right), to scale
5.0 Gm – Closest approach of Comet Halley to Earth, happened on 10 April 837
5.0 Gm – (proposed) Size of the arms of the giant triangle shaped Michelson interferometer of the Laser Interferometer Space Antenna (LISA) planned to start observations sometime in the 2030s.
Rigel and Aldebaran (top left and right) compared to smaller stars, the Sun (very small dot in lower middle, with orbit of Mercury as yellow ellipse) and transparent sphere with radius of one light minute.
From largest to smallest: Jupiter's orbit, red supergiant star Betelgeuse, Mars' orbit, Earth's orbit, star R Doradus, and orbits of Venus, Mercury. Inside R Doradus' depiction are the blue giant star Rigel and red giant star Aldebaran. The faint yellow glow around the Sun represents one light minute. Click image to see more details and links to their scales.
965 Gm – 6.4 AU – Maximum distance between the Earth and Jupiter
1 terametre
8 things in the terameter group
Comparison of size of the Kuiper belt (large faint torus) with the star VY Canis Majoris (within Saturn's orbit), Betelgeuse (inside Jupiter's orbit) and R Doradus (small central red sphere) together with the orbits of Neptune and Uranus, to scale. The yellow ellipses represent the orbits of each planet and the dwarf planet Pluto.
To help compare different distances, this section lists lengths starting at 10^{12}m (1 Tm or 1 billion km or 6.7 astronomical units).
1.4 Tm – 9.5 AU – Distance between Saturn and the Sun
1.83 Tm – 12.2 AU – Diameter of HR 5171 A, the largest known yellow hypergiant star although the latest research suggests it is a red hypergiant with a diameter about 2.1 Tm (14 AU)^{[136]}^{[137]}
1.5 Tm - 10 AU - Estimated diameter of VV Cephei A, a red supergiant.^{[138]}
2.9 Tm – 19.4 AU – Distance between Uranus and the Sun
4 Tm – 26.7 AU – Previous estimated diameter of VY Canis Majoris based on direct measurements of the radius at infrared wavelengths.^{[140]} The size was revised in 2012 through improved measurement techniques. (see above)^{[139]}
7.5 Tm – 50.1 AU – Outer radius of the Kuiper Belt, inner boundary of the Oort Cloud
10 terametres
Sedna's orbit (left) is longer than 100 Tm, but other lengths are between 10 and 100 Tm: Comet Hale-Bopp's orbit (lower, faint orange); one light-day (yellow spherical shell with yellow Vernal point arrow as radius); the heliosphere's termination shock (blue shell); and other arrows show positions of Voyager 1 (red) and Pioneer 10 (green). Click on image for larger view and links to other scales.
To help compare different distances this section lists lengths starting at 10^{13}m (10 Tm or 10 billion km or 67 astronomical units).
10 Tm – 67 AU – Diameter of a hypothetical Quasi-star
Largest circle with yellow arrow indicates one light year from Sun; Cat's Eye Nebula on left and Barnard 68 in middle are depicted in front of Comet 1910 A1's orbit. Click image for larger view, details and links to other scales.
7.5 Pm – 50,000 AU – Possible outer boundary of Oort cloud (other estimates are 75,000 to 125,000 or even 189,000 AU (1.18, 2, and 3 light years, respectively))
9.5 Pm – 63,241.1 AU – One light year, the distance travelled by light in one year
10 petametres
Objects with size order of magnitude 1e16m: Ten light years (94.6 Pm) radius circle with yellow Vernal Point arrow; Bubble Nebula (NGC 7635), left; Dumbbell Nebula (NGC 6853), right; one light year shell lower right with the smaller Cat's Eye Nebula (NGC_6543) and Barnard 68 adjacent.
1e16m lengths: Ten light years (94.6 Pm) yellow shell; Sirius below right; BL Ceti below left; Proxima and Alpha Centauri upper right; light year shell with Comet 1910 A1's orbit inside top right
To help compare different distances this section lists lengths starting at 10^{16}m (10 Pm or 66,800 AU, 1.06 light years).
15 Pm – 1.59 light years – Possible outer radius of Oort cloud
20 Pm – 2.11 light years – maximum extent of influence of the Sun's gravitational field
Lengths with order of magnitude 1e17m: yellow Vernal Point arrow traces hundred light year radius circle with smaller ten light year circle at right; globular cluster Messier 5 in background; 12 light year radius Orion Nebula middle right; 50 light year wide view of the Carina Nebula bottom left; Pleiades cluster and Bubble nebula with similar diameters each around 10 light years bottom right; grey arrows show distances from Sun to stars Aldebaran (65 light years) and Vega (25 light years).
To help compare different distances this section lists lengths between 10^{17}m (100 Pm or 11 light years) and 10^{18} m (106 light years).
260 Pm – 27 light years – Distance to Chara, a star approximately as bright as our Sun. Its faintness gives us an idea how our Sun would appear when viewed from even so close a distance as this.
Lengths with order of magnitude 1e18m: thousand light year radius circle with yellow arrow and 100 light year circle at right with globular cluster Messier 5 within and Carina Nebula in front; globular cluster Omega Centauri to left of both; part of the 1400 light year wide Tarantula Nebula fills the background.
This list includes distances between 1 and 10 exametres (10^{18} m). To help compare different distances this section lists lengths between 10^{18}m (1 Em or 105.7 light years) and 10^{19} m (1057 light years).
1.6 Em – 172 ± 12.5 light years – Diameter of Omega Centauri (one of the largest known globular clusters, perhaps containing over a million stars)^{[148]}^{[149]}
3.1 Em – 310 light years – Distance to Canopus according to Hipparcos^{[150]}
The universe within 1 billion light years of Earth
To help compare different orders of magnitude, this section lists distances starting at 10 Ym (10^{25}m or 1.1 billion light-years). At this scale, expansion of the universe becomes significant. Distance of these objects are derived from their measured redshifts, which depends on the cosmological models used.
13.7 Ym – 1.37 billion light-years – Length of the Sloan Great Wall
To help compare different orders of magnitude, this section lists distances starting at 100 Ym (10^{26}m or 11 billion light years). At this scale, expansion of the universe becomes significant. Distance of these objects are derived from their measured redshifts, which depend on the cosmological models used.
260 Ym – 27.4 billion light years – Diameter of the observable universe (double LTD)
440 Ym – 46 billion light years – Radius of the universe measured as a comoving distance
590 Ym – 62 billion light years – Cosmological event horizon: the largest comoving distance from which light will ever reach us (the observer) at any time in the future
886.48 Ym – 93.7 billion light years – The diameter of the observable universe; however, there might be unobserved distances that are even greater.
>1,000 Ym (1 kYm or xennameter in older usage) – Size of universe beyond the cosmic light horizon, depending on its curvature; if the curvature is zero (i.e. the universe is spatially flat), the value can be infinite (see Shape of the universe) as previously mentioned
Notes
^The diameter of human hair ranges from 17 to 181 μm Ley, Brian (1999). Elert, Glenn (ed.). "Diameter of a human hair". The Physics Factbook. Retrieved 8 December 2018.
^10^{115} is 1 followed by 115 zeroes, or a googol multiplied by a quadrillion. 10^{10115} is 1 followed by a quadrillion googol zeroes. 10^{1010122}is 1 followed by 10^{10122} (a googolplex^{10 sextillion}) zeroes.
^But not cloud or high-level fog droplets; droplet size increases with altitude. For a contradictory study indicating larger drop sizes even in ground fog, see Eldridge, Ralph G. (October 1961). "A Few Fog Drop-Size Distributions". Journal of Meteorology. 18 (5): 671–6. Bibcode:1961JAtS...18..671E. doi:10.1175/1520-0469(1961)018<0671:AFFDSD>2.0.CO;2.
^Carl R. Nave. "Neutron Absorption Cross-sections". Hyperphysics. Retrieved 4 December 2008. (area for 20 GeV about 10 × 10^{−42} m^{2} gives effective radius of about 2 × 10^{−21} m; for 250 GeV about 150 × 10^{−42} m^{2} gives effective radius of about 7 × 10^{−21} m)
^Abbott, B. P.; et al. (2016). "Observation of Gravitational Waves from a Binary Black Hole Merger". Physical Review Letters. 116 (6): 061102. arXiv:1602.03837. Bibcode:2016PhRvL.116f1102A. doi:10.1103/PhysRevLett.116.061102. PMID26918975. On 14 September 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10^{−21}.
^Flahaut E, Bacsa R, Peigney A, Laurent C (June 2003). "Gram-scale CCVD synthesis of double-walled carbon nanotubes". Chemical Communications. 12 (12): 1442–3. doi:10.1039/b301514a. PMID12841282.
^Stewart, Robert. "Dr". Radiobiology Software. Archived from the original on 30 June 2010. Retrieved 20 May 2015.
^Langevin, Dominique (2008). "Chapter 10: DNA-Surfactant/Lipid Complexes at Liquid Interfaces". In Dias, Rita S; Lindman, Bjorn (eds.). DNA Interactions with Polymers and Surfactants. Hoboken, NJ: John Wiley & Sons, Inc. p. 265. doi:10.1002/9780470286364.ch10. ISBN 978-0-470-25818-7. DNA has 20 elementary charges per helical turn over the corresponding length of 3.4nm
^Duncan, Martin (2008). "16"(PDF). Physics 216 – Introduction to Astrophysics. Archived from the original(PDF) on 17 December 2008. Retrieved 14 November 2008.
^Tegmark, M. (2003). "Parallel universes. Not just a staple of science fiction, other universes are a direct implication of cosmological observations". Scientific American. 288 (5): 40–51. doi:10.1038/scientificamerican0503-40. PMID12701329.
^Page, Don N.; Allende Prieto, C.; Garzon, F.; Wang, H.; Liu, C.; Deng, L. (2007). "Susskind's challenge to the Hartle Hawking no-boundary proposal and possible resolutions". Journal of Cosmology and Astro-Particle Physics. 2007 (1): 004. arXiv:hep-th/0610199. Bibcode:2007JCAP...01..004P. doi:10.1088/1475-7516/2007/01/004.
^Annis, Patty J. October 1991. Kansas State University. Fine Particle POLLUTION. Figure 1. (tobacco smoke: 10 to 1000 nm; virus particles: 3 to 50 nm; bacteria: 30 to 30000 nm; cooking oil smoke: 30 to 30000 nm; wood smoke: 7 to 3000 nm)
^Stryer, Lubert (1988). Biochemistry. San Francisco: W.H. Freeman. ISBN 978-0-7167-1843-7.
^Graham T. Smith (2002). Industrial metrology. Springer. p. 253. ISBN 978-1-85233-507-6.
^Eninger, Robert M.; Hogan, Christopher J.; Biswas, Pratim; Adhikari, Atin; Reponen, Tiina; Grinshpun, Sergey A. (2009). "Electrospray versus Nebulization for Aerosolization and Filter Testing with Bacteriophage Particles". Aerosol Science and Technology. 43 (4): 298–304. Bibcode:2009AerST..43..298E. doi:10.1080/02786820802626355.
^Gordon Ramel. "Spider Silk". Archived from the original on 4 December 2008. Retrieved 4 December 2008. garden spider silk has a diameter of about 0.003 mm ... Dragline silk (about 0.00032 inch (0.008 mm) in Nephila)
^ ^{a}^{b}Roberts, Richard W. (1 June 1975). Metric System of Weights and Measures - Guidelines for Use. USA: Director of the National Bureau of Standards. Federal Register FR Doc.75-15798 (1975-06-18). Accordingly, the following units and terms listed in the table of metric units in section 2 of the act of 28 July 1866, that legalized the metric system of weights and measures in the United States, are no longer accepted for use in the United States: myriameter, stere, millier or tonneau, quintal, myriagram, kilo (for kilogram).
^ ^{a}^{b}Judson, Lewis V. (1 October 1976) [1963]. "Appendix 7"(PDF). In Barbrow, Louis E. (ed.). Weights and Measures Standards of the United States, a brief history. Derived from a prior work by Louis A. Fisher (1905). USA: US Department of Commerce, National Bureau of Standards. p. 33. LCCN76-600055. NBS Special Publication 447; NIST SP 447; 003-003-01654-3. Retrieved 12 October 2015.
^Appell, Wolfgang (16 September 2009) [2002]. "Königreich Frankreich" [Kingdom of France]. Amtliche Maßeinheiten in Europa 1842 [Official units of measure in Europe 1842] (in German). Archived from the original on 5 October 2011. (Website based on Alte Meß- und Währungssysteme aus dem deutschen Sprachgebiet, ISBN 3-7686-1036-5)
^Brewster, David (1830). The Edinburgh Encyclopædia. 12. Edinburgh, UK: William Blackwood, John Waugh, John Murray, Baldwin & Cradock, J. M. Richardson. p. 494. Retrieved 9 October 2015.
^Brewster, David (1832). The Edinburgh Encyclopaedia. 12 (1st American ed.). Joseph and Edward Parker. Retrieved 9 October 2015.
^Dingler, Johann Gottfried (1823). Polytechnisches Journal (in German). 11. Stuttgart, Germany: J.W. Gotta'schen Buchhandlung. Retrieved 9 October 2015.
^Haugen, Einar, Norwegian English Dictionary, 1965, Oslo: Universitetsforlaget and Madison: University of Wisconsin Press, s.v. mil
^"FAQ-Alaska Highway Facts". The MILEPOST. Archived from the original on 29 September 2007. Retrieved 25 August 2007. 1,390 miles ... Alaska Route 2 and often treated as a natural extension of the Alaska Highway
^Neuroscience: The Science of the Brain"Archived copy". Archived from the original on 2 February 2011. Retrieved 8 June 2011.CS1 maint: Archived copy as title (link) p.44
^Moravveji, Ehsan; Guinan, Edward F; Shultz, Matt; Williamson, Michael H; Moya, Andres (4 January 2012). "Asteroseismology of the Nearby SN-II Progenitor: Rigel Part I. The MOST High Precision Photometry and Radial Velocity Monitoring". Astrophysical Journal. 747 (2): 2. arXiv:1201.0843. Bibcode:2012ApJ...747..108M. doi:10.1088/0004-637X/747/2/108.
^Richichi, A.; Roccatagliata, V.; Shultz, Matt; Williamson, Michael H.; Moya, Andres (2005). "Aldebaran's angular diameter: How well do we know it?". Astronomy & Astrophysics. 433 (1): 305–312. arXiv:astro-ph/0502181. Bibcode:2005A&A...433..305R. doi:10.1051/0004-6361:20041765. They derived an angular diameter of 20.58±0.03 milliarcsec, which given a distance of 65 light years yields a diameter of 61 million km.
^Chesneau, O.; Meilland, A.; Chapellier, E.; Millour, F.; Van Genderen, A. M.; Nazé, Y.; Smith, N.; Spang, A.; Smoker, J. V.; Dessart, L.; Kanaan, S.; Bendjoya, Ph.; Feast, M. W.; Groh, J. H.; Lobel, A.; Nardetto, N.; Otero, S.; Oudmaijer, R. D.; Tekola, A. G.; Whitelock, P. A.; Arcos, C.; Curé, M.; Vanzi, L. (2014). "The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in the common envelope phase". Astronomy & Astrophysics. 563: A71. arXiv:1401.2628v2. Bibcode:2014A&A...563A..71C. doi:10.1051/0004-6361/201322421.
^Wittkowski, M; Abellan, F. J; Arroyo-Torres, B; Chiavassa, A; Guirado, J. C; Marcaide, J. M; Alberdi, A; De Wit, W. J; Hofmann, K.-H; Meilland, A; Millour, F; Mohamed, S; Sanchez-Bermudez, J (28 September 2017). "Multi-epoch VLTI-PIONIER imaging of the supergiant V766 Cen: Image of the close companion in front of the primary". Astronomy & Astrophysics. 1709: L1. arXiv:1709.09430. Bibcode:2017A&A...606L...1W. doi:10.1051/0004-6361/201731569.
^ ^{a}^{b}Wittkowski, M.; Hauschildt, P.H.; Arroyo-Torres, B.; Marcaide, J.M. (5 April 2012). "Fundamental properties and atmospheric structure of the red supergiant VY CMa based on VLTI/AMBER spectro-interferometry". Astronomy & Astrophysics. 540: L12. arXiv:1203.5194. Bibcode:2012A&A...540L..12W. doi:10.1051/0004-6361/201219126.
^Monnier, J. D; Millan-Gabet, R; Tuthill, P. G; Traub, W. A; Carleton, N. P; Coudé Du Foresto, V; Danchi, W. C; Lacasse, M. G; Morel, S; Perrin, G; Porro, I. L; Schloerb, F. P; Townes, C. H (2004). "High-Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer". The Astrophysical Journal. 605 (1): 436–461. arXiv:astro-ph/0401363. Bibcode:2004ApJ...605..436M. doi:10.1086/382218.
^Parthasarathy, M. (2000). "Birth and early evolution of planetary nebulae". Bulletin of the Astronomical Society of India. 28: 217–224. Bibcode:2000BASI...28..217P.
^Michael Szpir (May – June 2001). "Bart Bok's Black Blobs". American Scientist. Archived from the original on 29 June 2003. Retrieved 19 November 2008. Bok globules such as Barnard 68 are only about half a light-year across and weigh in at about two solar masses
^Sandstrom, Karin M; Peek, J. E. G.; Bower, Geoffrey C.; Bolatto, Alberto D.; Plambeck, Richard L. (1999). "A Parallactic Distance of 389+24 −21 parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations". The Astrophysical Journal. 667 (2): 1161–1169. arXiv:0706.2361. Bibcode:2007ApJ...667.1161S. doi:10.1086/520922.
^diameter=sin(65 arcminutes)*1270 light years=24; where "65.00 x 60.0 (arcmin)" sourced from Revised NGC Data for NGC 1976
^distance × sin( diameter_angle ), using distance of 5kpc (15.8 ± 1.1 kly) and angle 36.3', = 172 ± 12.5 ly.
^van de Ven, G.; van den Bosch, R. C. E.; Verolme, E. K.; de Zeeuw, P. T. (2006). "The dynamical distance and intrinsic structure of the globular cluster ω Centauri". Astronomy and Astrophysics. 445 (2): 513–543. arXiv:astro-ph/0509228. Bibcode:2006A&A...445..513V. doi:10.1051/0004-6361:20053061. best-fit dynamical distance D=4.8±0.3 kpc ... consistent with the canonical value 5.0±0.2 kpc obtained by photometric methods
A centimetre (international spelling as used by the International Bureau of Weights and Measures; symbol cm) or centimeter (American spelling) is a unit of length in the metric system, equal to one hundredth of a metre, centi being the SI prefix for a factor of 1/100. The centimetre was the base unit of length in the now deprecated centimetre–gram–second (CGS) system of units.
Though for many physical quantities, SI prefixes for factors of 103—like milli- and kilo-—are often preferred by technicians, the centimetre remains a practical unit of length for many everyday measurements. A centimetre is approximately the width of the fingernail of an average adult person.
A decametre (International spelling as used by the International Bureau of Weights and Measures) or dekameter (American spelling), symbol dam (sometimes unofficially Dm or dkm), is a unit of length in the metric system equal to ten metres; it is very rarely used.This measure is included in the SI mostly for completeness: in principle, any combination of prefix and unit can be written, but many are rarely used in practice. One practical use of the decametre is for altitude of geopotential heights in meteorology. Meteorologists also use another seldom encountered SI prefix: hecto- in hectopascal (hPa). The volumetric form cubic decametre is convenient for describing large volumes of water such as in rivers and lakes. A cubic decametre (dam3) is equivalent to one megalitre (ML).
The square decametre (dam2), also known as the are (a), is the basis for the hectare (100 dam2), the standard metric unit of land registry.
The decimeter (SI symbol dm) is a unit of length in the metric system, equal to one tenth of a metre (the International System of Units base unit of length), ten centimetres or 3.937 inches.
The common non-SI metric unit of volume, the litre, is defined as one cubic decimetre (however, from 1901 to 1964, there was a slight difference between the two due to the litre being defined with respect to the kilogram rather than the metre).
Lorentz invariance follows from two independent postulates: the principle of relativity and the principle of constancy of the speed of light. Dropping the latter while keeping the former leads to a new invariance, known as Fock–Lorentz symmetry or the projective Lorentz transformation. The general study of such theories began with Fock, who was motivated by the search for the general symmetry group preserving relativity without assuming the constancy of c.
This invariance does not distinguish between inertial frames (and therefore satisfies the principle of relativity) but it allows for a varying speed of light in space, c; indeed it allows for a non-invariant c. According to Maxwell's equations, the speed of light satisfies
where ε_{0} and μ_{0} are the electric constant and the magnetic constant. If the speed of light depends upon the space–time coordinates of the medium, say x, then
$c(x)={\frac {1}{\sqrt {\chi (x)}}}\ ,$
where $\chi (x)$ represents the vacuum as a variable medium.
The hectometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: hm) or hectometer (American spelling) is an uncommonly used unit of length in the metric system, equal to one hundred metres. The word comes from a combination of "metre" and the SI prefix "hecto-", meaning "hundred". A soccer field is approximately 1 hectometre in length. The hectare (ha), a common metric unit for land area, is equal to one square hectometre (hm2).
The kilometre, (International spelling as used by the International Bureau of Weights and Measures; SI symbol: km; or ) or kilometer (American spelling) is a unit of length in the metric system, equal to one thousand metres (kilo- being the SI prefix for 1000). It is now the measurement unit used officially for expressing distances between geographical places on land in most of the world; notable exceptions are the United States and the road network of the United Kingdom where the statute mile is the official unit used.
k (pronounced ) is occasionally used in some English-speaking countries as an alternative for the word kilometre in colloquial writing and speech. A slang term for the kilometre in the US and UK military is klick.
Length is a measure of distance. In the International System of Quantities, length is any quantity with dimension distance. In most systems of measurement, the unit of length is a base unit, from which other units are derived.
In geometry, length is the most extended dimension of an object.Length may be distinguished from height, which is vertical extent, and width or breadth, which are the distance from side to side, measuring across the object at right angles to the length.
Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed).
In physics, length scale is a particular length or distance determined with the precision of one order of magnitude. The concept of length scale is particularly important because physical phenomena of different length scales cannot affect each other^{[citation needed]}^{[clarification needed]} and are said to decouple. The decoupling of different length scales makes it possible to have a self-consistent theory that only describes the relevant length scales for a given problem. Scientific reductionism says that the physical laws on the shortest length scales can be used to derive the effective description at larger length scales.
The idea that one can derive descriptions of physics at different length scales from one another can be quantified with the renormalization group.
In quantum mechanics the length scale of a given phenomenon is related to its de Broglie wavelength
$\ell =\hbar /p$ where $\hbar$ is the reduced Planck's constant and $p$ is the momentum that is being probed. In relativistic mechanics time and length scales are related by the speed of light. In relativistic quantum mechanics or relativistic quantum field theory, length scales are related to momentum, time and energy scales through Planck's constant and the speed of light. Often in high energy physics natural units are used where length, time, energy and momentum scales are described in the same units (usually with units of energy such as GeV).
Length scales are usually the operative scale (or at least one of the scales) in dimensional analysis. For instance, in scattering theory, the most common quantity to calculate is a cross section which has units of length squared and is measured in barns. The cross section of a given process is usually the square of the length scale.
The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 1012 km) or 5.88 trillion miles (5.88 x 1012 mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.
The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications. The unit most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).
The micrometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known by the previous name micron, is an SI derived unit of length equalling 1×10−6 metre (SI standard prefix "micro-" = 10−6); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.000039 inch).The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria, and for grading wool by the diameter of the fibres. The width of a single human hair ranges from approximately 10 to 200 μm. The longest human chromosome is approximately 10 μm in length.
The millimetre (international spelling as used by the International Bureau of Weights and Measures; SI unit symbol mm) or millimeter (American spelling) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre.
One millimetre is equal to 1000 micrometres or 1000000 nanometres. A millimetre is equal to exactly 5⁄127 (approximately 0.039370) of an inch, which is officially defined as exactly 25.4 millimetres.
A nautical mile is a unit of measurement used in both air and marine navigation, and for the definition of territorial waters. Historically, it was defined as one minute (1/60) of a degree of latitude. Today it is defined as exactly 1852 metres. The derived unit of speed is the knot, one nautical mile per hour.
The picometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: pm) or picometer (American spelling) is a unit of length in the metric system, equal to 1×10−12 m, or one trillionth (1/1000000000000) of a metre, which is the SI base unit of length.
The picometre is one thousandth of a nanometre, one millionth of a micrometre (also known as a micron), and used to be called micromicron, stigma, or bicron. The symbol µµ was once used for it. It is also one hundredth of an Ångström, an internationally recognised (but non-SI) unit of length.
Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3:
$1\,R_{\odot }=6.957\times 10^{5}{\hbox{ km}}$
695,700 kilometres (432,300 miles) is approximately 10 times the average radius of Jupiter, about 109 times the radius of the Earth, and 1/215th of an astronomical unit, the distance of the Earth from the Sun. It varies slightly from pole to equator due to its rotation, which induces an oblateness in the order of 10 parts per million.
In sciences such as physics, geography, astronomy, meteorology and statistics, the term scale or spatial scale is used for describing or classifying with large approximation the extent or size of a length, distance or area studied or described. For instance, in physics an object or phenomenon can be called microscopic if too small to be visible. In climatology, a micro-climate is a climate which might occur in a mountain, valley or near a lake shore, whereas in statistics a megatrend is a political, social, economical, environmental or technological trend which involves the whole planet or is supposed to last a very large amount of time.
In physics, the concept of scale is closely related to the more accurate concept of order of magnitude.
These divisions are somewhat arbitrary; where, on this table, mega- is assigned global scope, it may only apply continentally or even regionally in other contexts. The interpretations of meso- and macro- must then be adjusted accordingly.
A unit of length refers to any discrete, pre-established length or distance having a constant magnitude which is used as a reference or convention to express linear dimension. The most common units in modern use are U.S. customary units in the United States and metric units elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.
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