HOW DISPLACEMENT IS DETERMINED
 

Displacement magnitude is determined by mechanically measuring distance or by inferring it with observations and mathematical calculations. In the case of a car or truck driving along Route 52, displacement is measured with an odometer that counts the number of wheel rotations and multiplies this by the circumference of the wheel. In a laboratory environment, displacement magnitude can be measured with a meter stick, by triangulation, or by measuring the time it takes for a ray of light to travel between two points given the constancy of the speed of light (c ≈ 2.99792 × 10⁸ m/s).
The direction component of a displacement vector is determined by measuring one or more angles or coordinates relative to a reference axis. In the case of a local region on the Earth’s surface, direction can be found by specifying the azimuth, which is the angle clockwise relative to true north. This is the scheme used by hikers and backpackers. In three-dimensional space, direction angles are used. A reference axis, for example, a vector pointing toward Polaris, the North Star, is defined. Then two angles are specified in a coordinate system based on this axis. The most common system used by astronomers and space scientists involves angles called celestial latitude and celestial longitude or, alternatively, right ascension and declination. (If this is not familiar to you, and if you didn’t see fit to study Part Zero, this might be a good time to reconsider that decision!)