Science Fair Project Encyclopedia
- This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms). In architecture, apsis is a synonym for apse; Apogee is also the name of a video game publisher.
In astronomy, an apsis (plural apsides "ap-si-deez") is the point of greatest or least distance of the elliptical orbit of a celestial body from its center of attraction (the center of mass of the system).
The point of closest approach is called the periapsis and the point of farthest approach is the apoapsis. A straight line drawn through the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse, the line through the longest part of the ellipse.
Related terms are used to identify the body being orbited. The most common are perigee and apogee, referring to earth orbits, and perihelion and aphelion, referring to orbits around the sun.
- Periapsis: maximum speed at minimum distance (periapsis distance)
- Apoapsis: minimum speed at maximum distance (apoapsis distance)
where one easily verifies
- is the semi-major axis
- is the eccentricity
- is the specific relative angular momentum
- is the specific orbital energy
- is the standard gravitational parameter
Note that for conversion from heights above the surface to distances, the radius of the central body has to be added, and conversely.
The geometric mean of the two speeds is , the speed corresponding to a kinetic energy which, at any position of the orbit, added to the existing kinetic energy, would allow the orbiting body to escape (the square root of the sum of the squares of the two speeds is the local escape velocity).
Various related terms are used for other celestial objects. 'Perigee', 'perihelion' and 'periastron' and corresponding 'apo-' forms are very frequently used in the astronomical literature, while '-lune', '-selene', '-cytherion', '-areion' and '-jove' are very occasionally used; other terms are never used in practice, with 'perigee' being quite commonly used as a generic 'closest approach to planet' term instead of specifically applying to the Earth.
|Body||Closest approach||Farthest approach|
(1) These terms are properly pronounced 'affelion', 'affermion' and 'affadion', but in practice, 'ap-helion' is commonly heard, while the latter two terms are not used.
Since "peri" and "apo" are Greek, it is considered more correct to use the Greek form for the body. In practice, the '-selene' and '-lune' forms are both used, albeit very infrequently, like the '-cynthion' form, which is reserved for artificial bodies. For Jupiter, the '-jove' form is occasionally seen whilst the '-zene' form is essentially never used, like '-hermion', '-krone', '-uranion', '-poseidion' and '-hadion'. The '-saturnium' form was occasionally used in the late 19th century and early 20th century (see these two examples: 2 and 3).
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