Dictionary

Apsis

Points on the elliptical orbit of a planet or other heavenly body around a central star. The apses are the two main vertices of the ellipse. Here the perihelion is the point with the shortest distance to the focal point. The aphelion, the point with the greatest distance to the focal point.

Definitioner

star

A cosmic gas ball that is massive enough for pressure and temperatur in its core to reach values where self-sustained nuclear fusion reactions set in. The energy set free in these reactions makes stars into very bright sources of light and other forms of electromagnetic radiation. Once the nuclear fuel is exhausted, the star becomes a white dwarf, a neutron star or a black hole.

point

Elementary "building block" of geometrical entities such as surfaces or more general spaces. For instance, a surface is the set of all its points, of all possible locations on the surface, and all geometrical objects in that surface are defined by the points that belong to them - for instance, a line on the surface is the set of (infinitely many) points.

planet

Planets are not-too-small companions of a star that are not stars themselves (nor ever were stars). In our solar system, the planets are, listed from the one closest to the sun to the one farthest: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. As of August 2006, Pluto, which used to be a proper planet, is officially a "dwarf planet". In the night sky, the distinguishing characteristic of planets is that they move around relative to the unchanging background of stars - which gave them their name, loosely translated from Greek as "wanderers".

ellipse

An ellipse is a planar curve. All points on the curve are located around two focal points, such that the sum of the distances to each focal point is equal for all points. Special cases of an ellipse are a circle (in this special case, both focal points are at the same location) and a straight line joining the two focal points (in this special case, the specific distance chosen is equal to the distance between the two focal points). As far as gravity is concerned, ellipses are of interest as the orbit of a lonely planet around a central star is an ellipse, according to Newton's theory of gravity.

Dictionary

Apsis

Further Articles

Tag Cloud