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Because the orbits of planets, asteroids and comets around the Sun are not perfect circles, each of these bodies has a point in its orbit where it is at its farthest possible distance from the Sun, a point known as its aphelion. Similarly, each body will have a closest possible point of approach, opposite the aphelion, known as the perihelion.

The difference in distance between the aphelion and perihelion points will vary according to the nature of the orbit in question, and in particular its eccentricity, a value describing the degree of elongation of the orbit's ellipse. For example, Earth has relatively low eccentricity, and hence an orbit that is close to a circle in form. Earth reaches its aphelion point in early July each year, when it falls some 152.1 million kilometres from the Sun (at its perihelion in January, it is five million kilometres closer).

Bodies with more eccentric orbits, the aphelion can be much greater than the perihelion. For example, at its aphelion point the dwarf planet Pluto reaches a distance of 49.3 AU (7.4 billion kilometres) from the Sun. By comparison, its perihelion distance is just 29.7 AU (4.4 billion kilometres) which actually brings it within the orbit of the next innermost planet, Neptune.

The term aphelion applies specifically to bodies that are in orbit around the Sun. The more general term for any body orbiting any other is apoapsis or apoapse. For objects orbiting stars other than the Sun, the term apastron (or occasionally apoastron) is used to describe their most distant point from the star.


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