اختلاف المنظر الشمسي

اختلاف المنظر الشمسي solar parallax مصطلح فلكي يشير الى نصف القطر الزاوي لخط الإستواء الأرضي كما يرى من مسافة وحدة فلكية ويساوي 8,794 ثانية قوسية.

After Copernicus proposed his heliocentric system, with the Earth in revolution around the Sun, it was possible to build a model of the whole Solar System without scale. To ascertain the scale, it is necessary only to measure one distance within the Solar System, e.g., the mean distance from the Earth to the Sun (now called an astronomical unit, or AU). When found by triangulation, this is referred to as the solar parallax, the difference in position of the Sun as seen from the Earth's center and a point one Earth radius away, i.e., the angle subtended at the Sun by the Earth's mean radius. Knowing the solar parallax and the mean Earth radius allows one to calculate the AU, the first, small step on the long road of establishing the size and expansion age[1] of the visible Universe.

A primitive way to determine the distance to the Sun in terms of the distance to the Moon was already proposed by Aristarchus of Samos in his book On the Sizes and Distances of the Sun and Moon. He noted that the Sun, Moon, and Earth form a right triangle (with the right angle at the Moon) at the moment of first or last quarter moon. He then estimated that the Moon–Earth–Sun angle was 87°. Using correct geometry but inaccurate observational data, Aristarchus concluded that the Sun was slightly less than 20 times farther away than the Moon. The true value of this angle is close to 89° 50', and the Sun is about 390 times farther away.[2]

Aristarchus pointed out that the Moon and Sun have nearly equal apparent angular sizes, and therefore their diameters must be in proportion to their distances from Earth. He thus concluded that the Sun was around 20 times larger than the Moon. This conclusion, although incorrect, follows logically from his incorrect data. It suggests that the Sun is larger than the Earth, which could be taken to support the heliocentric model.[3]

Measuring Venus transit times to determine solar parallax

Although Aristarchus' results were incorrect due to observational errors, they were based on correct geometric principles of parallax, and became the basis for estimates of the size of the Solar System for almost 2000 years, until the transit of Venus was correctly observed in 1761 and 1769.[2] This method was proposed by Edmond Halley in 1716, although he did not live to see the results. The use of Venus transits was less successful than had been hoped due to the black drop effect, but the resulting estimate, 153 million kilometers, is just 2% above the currently accepted value, 149.6 million kilometers.

Much later, the Solar System was "scaled" using the parallax of asteroids, some of which, such as Eros, pass much closer to Earth than Venus. In a favorable opposition, Eros can approach the Earth to within 22 million kilometers.[4] During the opposition of 1900–1901, a worldwide program was launched to make parallax measurements of Eros to determine the solar parallax[5] (or distance to the Sun), with the results published in 1910 by Arthur Hinks of Cambridge[6] and Charles D. Perrine of the Lick Observatory, University of California.[7]

Perrine published progress reports in 1906[8] and 1908.[9] He took 965 photographs with the Crossley Reflector and selected 525 for measurement.[10] A similar program was then carried out, during a closer approach, in 1930–1931 by Harold Spencer Jones.[11] The value of the Astronomical Unit (roughly the Earth-Sun distance) obtained by this program was considered definitive until 1968, when radar and dynamical parallax methods started producing more precise measurements.

Also radar reflections, both off Venus (1958) and off asteroids, like Icarus, have been used for solar parallax determination. Today, use of spacecraft telemetry links has solved this old problem. The currently accepted value of solar parallax is 8.794143 arcseconds.[12]

انظر أيضا

المصادر

  • مؤمن, عبد الأمير (2006). قاموس دار العلم الفلكي. بيروت، لبنان: دار العلم للملايين. {{cite book}}: Cite has empty unknown parameter: |طبعة أولى coauthors= (help)
  1. ^ Freedman, W.L. (2000). "The Hubble constant and the expansion age of the Universe". Physics Reports. 333 (1): 13–31. arXiv:astro-ph/9909076. Bibcode:2000PhR...333...13F. doi:10.1016/S0370-1573(00)00013-2. S2CID 413222.
  2. ^ أ ب خطأ استشهاد: وسم <ref> غير صحيح؛ لا نص تم توفيره للمراجع المسماة Gutzwiller
  3. ^ Al-Khalili, Jim (2010), Pathfinders: The Golden Age of Arabic Science, Penguin UK, p. 270, ISBN 9780141965017, https://books.google.com/books?id=ntLEUHTXxUMC&pg=PT270, "Some have suggested that his calculation of the relative size of the earth and sun led Aristarchus to conclude that it made more sense for the earth to be moving around the much larger sun than the other way round." 
  4. ^ Whipple 2007, p. 47.
  5. ^  Newcomb, Simon (1911). "Parallax" . In Chisholm, Hugh (ed.). دائرة المعارف البريطانية. Vol. 20 (eleventh ed.). Cambridge University Press. p. 761. {{cite encyclopedia}}: Cite has empty unknown parameter: |coauthors= (help)
  6. ^ Hinks, Arthur R. (1909). "Solar Parallax Papers No. 7: The General Solution from the Photographic Right Ascensions of Eros, at the Opposition of 1900". Monthly Notices of the Royal Astronomical Society. 69 (7): 544–67. Bibcode:1909MNRAS..69..544H. doi:10.1093/mnras/69.7.544.
  7. ^ Perrine, Charles D. (1910). Determination of the solar parallax from photographs of Eros made with the Crossley reflector of the Lick Observatory University of California (First ed.). Washington, D. C.: Carnegie Institution of Washington. pp. 1–104.
  8. ^ Perrine, C. D. (1906). "The Measurement and Reduction of the Photographs of Eros Made With the Crossley Reflector in 1900". Publications of the Astronomical Society of the Pacific. 18 (10): 226.
  9. ^ Perrine, Charles D. (1908). "Progress on the Crossley Eros Solar Parallax Work". Publications of the Astronomical Society of the Pacific. 20 (120): 184. Bibcode:1908PASP...20..184P. doi:10.1086/121816. S2CID 121782316.
  10. ^ Campbell, W. W. (1906). "Reports of the Observatories: Lick Observatory". Publications of the Astronomical Society of the Pacific. 19 (113): 92.
  11. ^ Jones, H. Spencer (1941). "The Solar Parallax and the Mass of the Moon from Observations of Eros at the Opposition of 1931". Mem. Roy. Astron. Soc. 66: 11–66.
  12. ^ "Astronomical Constants" (PDF). US Naval Observatory. Archived from the original (PDF) on 2011-07-20.
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