Oortn.
/ɔːt/, /ʊət/, U.S. /ɔrt/ Astronomy.
1. Oort constant n. (also Oort's constant) either of two constants occurring in a pair of equations relating the radial and tangential velocities of a star in the Galaxy to its distance from the sun.The two constants are denoted by A and B. A is a measure of the shear in the Galactic disc in the neighbourhood of the sun, and is about 15 km s−1 per kiloparsec; B measures the vorticity of the material of the disc in the solar neighbourhood, and is about −10 km s−1 per kiloparsec.
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the world > the universe > cosmology > science of observation > astronomical calculation > [noun] > part
argumentc1405
astronomicalsa1690
element1788
parameter1829
perturbative function1856
luminosity function1924
Hubble constant1933
Oort constant1941
1941 B. J. Bok & P. F. Bok Milky Way v. 106 The ‘Oort’ constant..measures the maximum effect [of differential rotation] at a standard distance.
1977 J. Narlikar Struct. Universe iii. 62 Oort expressed the transverse velocity of the star in the following form T = r{B + A cos 2(l−l0)}. Here r is the distance of the star from the Sun, l its galactic longitude, and l0 is the galactic longitude of the galactic centre. A and B are constants called Oort's constants.
1994 H. Karttunen et al. Fund. Astron. (ed. 2) xviii. 394 If the distance to the stars involved is known, the amplitude of the curve determines the value of the Oort constant A.
2. Oort cloud n. (also Oort's cloud) a cloud of icy bodies postulated to surround the solar system well beyond the orbit of Pluto and to act as a reservoir of comets (now usually identified with the Kuiper belt). Formerly also Oort's comet cloud.
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the world > the universe > constellation > comet or meteor > comet > [noun] > group of
Oort cloud1968
1968 D. C. Knight Comets 38 Fred L. Whipple recently proposed that a yet-to-be-seen comet belt, distinct from Oort's ‘comet cloud’, lies near the orbit of Pluto.
1976 National Observer (U.S.) 24 Apr. 6/1 Van Flandern's work also casts doubt on ‘Oort's Cloud’, one of the more cherished concepts of comet studies.
1986 J. Allman Scenarios for Mixed Landscape 30 Their multi-eyed cubist heads tilted towards interstices of air, perhaps it was then the sun's sister entered the Oort cloud.
1987 M. W. Bonanno Strangers from Sky i. xi. 389 Static from the Oort Cloud swallowed the signal.
2000 Wired Jan. 71 Controlled fusion will make possible access all the way to the outer solar system and the Oort cloud.
3. Oort limit n. (also Oort's limit) the value of the local mass density in the part of the galactic disc near the sun that is required to account for the observed gravitational field.
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1977 Astron. Jrnl. 82 195/1 The term Oort limit is used to designate the mass density in the solar neighborhood necessary to explain the observed motions of stars perpendicular to the galactic plane.
1978 R. J. Tayler Galaxies vi. 137 This value of the density of matter near the galactic plane is known as Oort's limit.
1992 Independent 23 Nov. 20/3 In 1932 he [sc. Oort] showed that the motions of stars perpendicular to the galactic disc implied a mass of gravitating matter in the disc, the Oort limit, which was significantly in excess of the known mass of stars.
This entry has been updated (OED Third Edition, June 2004; most recently modified version published online March 2022).
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n.1941