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In astronomy, galactocentrism is the theory that the Milky Way Galaxy, home of Earth ' s Solar System, is at or near the center of the Universe. [1] [2]Thomas Wright and Immanuel Kant first speculated that fuzzy patches of light called nebulae were actually distant "island universes" consisting of many stellar systems. [3]
He waited until 1802 (in Catalogue of 500 new Nebulae, nebulous Stars, planetary Nebulae, and Clusters of Stars; with Remarks on the Construction of the Heavens) to announce the hypothesis that the two stars were "binary sidereal systems" orbiting under mutual gravitational attraction, a hypothesis he confirmed in 1803 in his Account of the ...
The Solar System travels alone through the Milky Way in a circular orbit approximately 30,000 light years from the Galactic Center. Its speed is about 220 km/s. The period required for the Solar System to complete one revolution around the Galactic Center, the galactic year, is in the range of 220–250 million years. Since its formation, the ...
Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them. [25] Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called dark nebulae. [25]
1750 – Thomas Wright discusses galaxies and the flattened shape of the Milky Way and speculates nebulae as separate. [11] 1755 – Immanuel Kant drawing on Wright's work conjectures that our galaxy is a rotating disk of stars held together by gravity, and that the nebulae are separate such galaxies; he calls them Island Universes.
[2] [21] [62] [66] In the Solar System they may be represented by Earth and Venus. [21] Formation of both planets required merging of approximately 10–20 embryos, while an equal number of them were thrown out of the Solar System. [62] Some of the embryos, which originated in the asteroid belt, are thought to have brought water to Earth. [64]
In 1917, Heber Doust Curtis observed a nova within what then was called the "Andromeda Nebula". Searching the photographic record, 11 more novae were discovered. Curtis noticed that novas in Andromeda were drastically fainter than novas in the Milky Way. Based on this, Curtis was able to estimate that Andromeda was 500,000 light-years away.
1796 – Pierre Laplace re-states the nebular hypothesis for the formation of the Solar System from a spinning nebula of gas and dust. [ 124 ] 1798 – Henry Cavendish accurately measures the gravitational constant in the laboratory , which allows the mass of the Earth to be derived, and hence the masses of all bodies in the Solar System.