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# Other Coordinate Systems for Physics Help

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## Introduction

Here are some other coordinate systems that you are likely to encounter in your journeys through the world of physics. Keep in mind that the technical details are simplified for this presentation. As you gain experience using these systems, you will be introduced to more details, but they would confuse you if we dealt with them now.

## Latitude And Longitude

Latitude and longitude angles uniquely define the positions of points on the surface of a sphere or in the sky. The scheme for geographic locations on the earth is illustrated in Fig. 3-14 a . The polar axis connects two specified points at antipodes on the sphere. These points are assigned latitude +90 (north pole) and −90 (south pole). The equatorial axis runs outward from the center of the sphere at a right angle to the polar axis. It is assigned longitude 0. Latitude is measured positively (north) and negatively (south) relative to the plane of the equator. Longitude is measured counterclockwise (east) and clockwise (west) relative to the equatorial axis. The angles are restricted as follows:

Fig. 3-14 . (a) Latitude and longitude on the earth are measured in degrees.

−90° ≤ latitude ≤ +90°

−180° ≤ longitude ≤ +180°

On the earth’s surface, the half-circle connecting the zero-longitude line with the poles passes through Greenwich, England, and is known as the Greenwich meridian or the prime meridian . Longitude angles are defined with respect to this meridian.

Fig. 3-14 . (b) Declination (dec) and right ascension (RA) are used to find coordinates in the heavens.

## Celestial Coordinates

Celestial latitude and celestial longitude are extensions of the earth’s latitude and longitude into the heavens. An object whose celestial latitude and longitude coordinates are ( x, y ) appears at the zenith (straight overhead) in the sky from the point on the earth’s surface whose latitude and longitude coordinates are ( x, y ).

Declination and right ascension define the positions of objects in the sky relative to the stars. Figure 3-14 b applies to this system. Declination (abbreviated dec ) is identical to celestial latitude. Right ascension (abbreviated RA ) is measured eastward from the vernal equinox (the position of the sun in the heavens at the moment spring begins in the northern hemisphere). Right ascension is measured in hours (symbolized h ) rather than degrees, where there are 24h in a 360° circle. The angles are restricted as follows:

−90° ≤ dec ≤ +90°

0h ≤ RA < 24h

## Cartesian Three-space

An extension of rectangular coordinates into three dimensions is cartesian three-space (Fig. 3-15), also called rectangular three-space or xyz space . Independent variables are usually plotted along the x and y axes; the dependent variable is plotted along the z axis. “Graphs” of this sort show up as snakelike curves winding and twisting through space or as surfaces such as spheres, ellipsoids, or those mountain-range-like displays you have seen in the scientific magazines. Usually, the scales are linear; that is, the increments are the same size throughout each scale. However, variations of these schemes can employ nonlinear graduations for one, two, or all three scales.

Fig. 3-15 . Cartesian three-space, also called rectangular three-space or xyz space .

Computers are invaluable in graphing functions in rectangular three-space. Computers can show perspective, and they let you see the true shape of a surface plot. A good three-dimensional (3D) graphics program lets you look at a graph from all possible angles, even rotating it or flipping it over in real time.

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