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Apogee is also the name of a major video game publisher.
elements of an orbit
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 centre of attraction (the centre 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.
We have:
where one easily verifies
(each the same for both points, like they are for the whole orbit, in accordance with Kepler's laws of planetary motion (conservation of angular momentum) and the conservation of energy)
where:
Properties:
il||100}}Note that for conversion from heights above the surface to distances, the radius of the central body has to be added, and conversely.
The arithmetic mean of the two distances is the semi-major axis . The geometric mean of the two distances is the semi-minor axisIn geometry, the semi-minor axis (also semiminor axis applies to ellipses and hyperbolas. Ellipse The semi-minor axis of an ellipse is one half of the minor axis, running from the center, halfway between and perpendicular to the line running between the f .
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).
Related and increasingly specific terms are often used to identify the body being orbited. For example, the terms apogee and perigee refer to an orbit around the earth, aphelion and perihelion refer to an orbit around the sun, and apastron and periastron refer to an orbit around a star. Other terms are less commonly or rarely used:
| Body | Closest approach | Farthest approach |
|---|---|---|
| StarFor alternate meanings see star (disambiguation Hubble Space Telescope of the Sagittarius Star Cloud in the Milky Way Galaxy. A star is any massive gaseous celestial body in outer space. Stars appear as shining points in the nighttime sky that twinkle bec | Periastron | Apastron |
| Black holeThis article is about the astronomical body. For other uses, see Black hole (disambiguation). roche limit. Infalling matter forms an accretion disk, with some of the matter being ejected in highly energetic polar jets. A black hole is a concentration of m | Perimelasma | Apomelasma |
| SunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt | Perihelion | Aphelion (1) |
| MercuryMercury is the closest planet to the Sun, and the second-smallest planet in the solar system. Mercury ranges from 0. 5 in apparent magnitude; Mercury is sufficiently "close" to the Sun that telescopes rarely examine it (the greatest elongation is 28. | Perihermion | Aphermion (2) |
| VenusVenus is the second planet from the Sun, named after the Roman goddess Venus. It is a terrestrial planet, very similar in size and bulk composition to Earth; it is sometimes called Earth's "sister planet" as a result of this similarity. Although all plane | Pericytherion | Apocytherion |
| EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla | Perigee | Apogee |
| MoonFor other moons in the solar system see natural satellite. For other uses see Moon (disambiguation). The Moon is the only natural satellite of Earth. It has no formal name other than "The Moon" although it is occasionally called Luna ( Latin for moon to d | Periselene | Aposelene (3) |
| Mars | Periareion | Apoareion |
| Jupiter | Perizene | Apozene (4) |
| Saturn | Perikrone | Apokrone |
| Uranus | Periuranion | Apuranion |
| Neptune | Periposeidion | Apoposeidion |
| Pluto | Perihadion | Aphadion (5) |
The terms are formed from the Greek roots for the planet names rather than the Latin ones, since "peri" and "apo" are Greek and it is considered bad form to mix Greek and Latin roots.
(1) Pronounced "Ap-helion", not "Aff-elion".
(2) Pronounced "Ap-hermion", not "Aff-ermion".
(3) Perilune/Apolune are to be avoided. Pericynthion/Apocynthion are sometimes used for artificial bodies.
(4) In theory, Perijove/Apojove are to be avoided as they mix Greek and Latin roots. In practice, however, perijove and apojove are widely used and are more recognizable than perizene/apozene.
(5) Pronounced "Ap-hadion", not "Aff-adion".
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