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| | Coordinate Systems |
 | | The numbers assigned to a position, or point, are called the coordinates of that point (in the coordinate system under consideration). |
 | | In physical laboratories, and sometimes for everyday purpose, it is convenient to use cartesian coordinates, i.e., just length, width and height measured from a reference point (the origin of the system) in three mutually othogonal (or perpendicular) directions, i.e., three straight lines called axes of the coordinate system, which have mutually right angles between them. |
 | | Each coordinate system is now uniquely determined by its origin, either its polar axis or the equatorial plane (the other is always perpendicular and thus given by the one), and the reference direction. |
| www.seds.org /~spider/spider/ScholarX/coord_bas.html (630 words) |
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