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Because the observable universe is defined as that region of the Universe visible to terrestrial observers, Earth is, because of the constancy of the speed of light, the center of Earth's observable universe. Reference can be made to the Earth's position with respect to specific structures, which exist at various scales. It is still ...
Coordinate systems in astronomy can specify an object's relative position in three-dimensional space or plot merely by its direction on a celestial sphere, if the object's distance is unknown or trivial. Spherical coordinates, projected on the celestial sphere, are analogous to the geographic coordinate system used on the surface of Earth.
[292] [293] Earth was generally believed to be the center of the universe until the 16th century, when scientists first concluded that it was a moving object, one of the planets of the Solar System. [ 294 ]
The comoving distance from Earth to the edge of the observable universe is about 14.26 gigaparsecs (46.5 billion light-years or 4.40 × 10 26 m) in any direction. The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs [27] (93 billion light-years or 8.8 × 10 26 m). [28]
geometric position The position of an object (celestial or otherwise) with respect to the center of the Earth or to the position of an observer, i.e. as defined by a straight line between the center of the Earth (or the observer) and the object at a given time, without any corrections for light-time, aberration, etc. [14] geostationary orbit
The observable universe contains as many as an estimated 2 trillion galaxies [95] [96] [97] and, overall, as many as an estimated 10 24 stars [98] [99] – more stars (and earth-like planets) than all the grains of beach sand on planet Earth; [100] [101] [102] but less than the total number of atoms estimated in the universe as 10 82; [103] and ...
The ordered set { t} is called a time axis. Motion (also path or trajectory) is a function r : Δ → R 3 that maps a point in the interval Δ from the time axis to a position (radius vector) in R 3. The above four concepts are the "well-known" objects mentioned by Isaac Newton in his Principia:
Because astronomical objects are at such remote distances, casual observation of the sky offers no information on their actual distances. All celestial objects seem equally far away, as if fixed onto the inside of a sphere with a large but unknown radius, [1] which appears to rotate westward overhead; meanwhile, Earth underfoot seems to remain still.