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The Milky Way, or "milk circle", was just one of 11 "circles" the Greeks identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, the Arctic Circle and the Antarctic Circle, and two colure circles passing through both poles.
Barnard's Star is the star with the highest proper motion. [ 1 ] In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space. Stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as the internal kinematics of more ...
The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. [1][2] Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, [3][4][5] a compact radio source which is almost exactly at the galactic rotational center ...
The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane parallel to an approximation of the galactic plane but offset to its north. It uses the right-handed convention ...
Galactic quadrant. Longitudinal lines of the galactic coordinate system. A galactic quadrant, or quadrant of the Galaxy, is one of four circular sectors in the division of the Milky Way Galaxy. Numbered quadrants and sectors of constellations. Quadrants as starcharts, with most prominent stars marked.
Satellite galaxies that orbit from 1,000 ly (310 pc) of the edge of the disc of the Milky Way Galaxy to the edge of the dark matter halo of the Milky Way at 980,000 ly (300 kpc) from the center of the galaxy, [ a ] are generally depleted in hydrogen gas compared to those that orbit more distantly. This is because of their interactions with the ...
Milky Way with galactic plane, ecliptic and celestial equator. The galactic plane is the plane on which the majority of a disk-shaped galaxy 's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms galactic plane and galactic poles usually refer specifically to the plane and poles ...
The path-line is the combined motion of the planet's orbit (deferent) around Earth and within the orbit itself (epicycle). Around 210 BCE, Apollonius of Perga shows the equivalence of two descriptions of the apparent retrograde planet motions (assuming the geocentric model), one using eccentrics and another deferent and epicycles . [ 43 ]