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Knowledge of the location of Earth has been shaped by 400 years of telescopic observations, and has expanded radically since the start of the 20th 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 ]
Due to forces that the Sun and Moon exert, Earth's equatorial plane moves with respect to the celestial sphere. Earth rotates while the ECI coordinate system does not. Earth-centered inertial (ECI) coordinate frames have their origins at the center of mass of Earth and are fixed with respect to the stars. [1] "
The yellow dot indicates the position of the Sun, the red dots the position of the embedded clusters, which serve as indicators of the spiral structure It is challenging to ascertain the presence of spiral arms in the disc of our galaxy through optical observation, given that the Sun is situated within the plane of the Milky Way disc, and the ...
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The arm is named after the Orion Constellation, one of the most prominent constellations of the Northern Hemisphere in winter (or the Southern Hemisphere in summer).Some of the brightest stars in the sky as well as other well-known celestial objects of the constellation (e.g. Betelgeuse, Rigel, the three stars of Orion's Belt, and the Orion Nebula) are found within it, as shown on Orion Arm's ...
Earth's rotational velocity also varies in a phenomenon known as length-of-day variation. [172] Earth's annual orbit is elliptical rather than circular, and its closest approach to the Sun is called perihelion. In modern times, Earth's perihelion occurs around 3 January, and its aphelion around 4 July.
The spokes of that "star" are marked with constellation abbreviations, "Cas" for "Cassiopeia", etc. The spiral arms are colored differently in order to highlight what structure belongs to which arm. H II regions are marked as dots colored in the same color as their spiral arm. They come in three sizes, measured by the excitation parameter U:
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