Relative size

The Moon is exceptionally large relative to the Earth: a quarter the diameter of the planet and 1/81 its mass.[46] It is the largest moon in the Solar System relative to the size of its planet, though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass.[91] However, the Earth and Moon are still considered a planetsatellite system, rather than a double-planet system, as their barycentre, the common centre of mass, is located 1,700 km (about a quarter of the Earth's radius) beneath the surface of the Earth.[92]A dwarf planet is a planetary-mass object that is neither a planet nor a satellite. More explicitly, the International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun[1] that is massive enough for its shape to be controlled by gravitational rather than mechanical forces (that is, it has sufficient mass to overcome its internal compressive strength and achieve hydrostatic equilibrium, and is thus an ellipsoid in shape), but that unlike a planet has not cleared its orbital region of other objects.[2][3] The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,[1] brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris.[4] This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it w s said to be the "right decision" by Mike Brown,[5][6][7] who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern,[8][9] who had coined the term dwarf planet in 1990.[10] The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake, and Eris.[11] However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ?838 km assuming a geometric albedo of ?1)[12] are to be named under the assumption that they are dwarf planets.[13] The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.In astronomy, barycentric coordinates are non-rotating coordinates with origin at the center of mass of two or more bodies. The barycenter (or barycentre; from the Greek -? heavy + ?- centre + -ic[1]) is the point between two objects where they balance each other. For example, it is the center of mass where two or more celestial bodies orbit each other. When a moon orbits a planet, or a planet orbits a star, both bodies are actually orbiting around a point that is not at the center of the primary (the larger body). For example, the Moon does not orbit the exact center of the Earth, but a point on a line between the center of the Earth and the Moon, approximately 1,710 km below the surface of the Earth, where their respective masses balance. This is the point about which the Earth and Moon orbit as they travel around the Sun.