Planetary Earth

A sphere (planetary) is a reduced representation and in three dimensions, of a roughly spherical celestial body like a Planet, a star, a natural satellite (lunar sphere, for example), and in particular the Ground (Globe). They can also be a spherical representation of the sky and stars, other than the Sun, the Moon and planets because of their variable relative positions; however, the “orbit” of the Sun is indicated: it is a celestial Globe. One speaks thus about “pair of spheres” including terrestrial and celestial.

The terrestrial spheres are the only geographical representation of the Ground which does not display a geometrical distorsion. The spheres as the Ground are charted on a plane surface by using a cartographic projection which inevitably displays a distorsion. These projections can preserve either the angles, or surfaces. The scale of representation of a terrestrial sphere is typically of about a 1/40 millionth.

The Ground is obviously the planet most represented, but of the spheres of the Sun, of the the Moon exist. Spheres were carried out for other celestial bodies, including for fictitious celestial bodies.

The oldest terrestrial sphere arrived to us is the sphere of the German Cartographe Martin Behaim (towards 1492).

A planetary sphere is generally assembled on a tilted axis. For more convenience, the slope of the presentation is in conformity with that the axis of rotation of planet with respect to its Sun. This makes it possible to easily visualize the days and the seasons. Sometimes the sphere presents a relief appearing topography; in the case of the Ground, rises are accentuated, without what they would be not easily detectable.

In addition to the purely decorative aspect, the spheres are at the origin of the instruments of work for the navigators and the astronomers, in particular. They quickly become educational tools and decorate the classrooms of the elementary schools traditionally. The Germans even produced as of the extreme end of the 18th century of the dumb spheres, without any legend, in order to question the pupils.

See too

Simple: Earth

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