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| Image generated by AI using Bing Image Creator |
The Solar System and the Earth within it is moving around the Milky
Way galaxy at a velocity of 826,000 km/h. The human body can't sense
constant speed, only acceleration when we're speeding up or slowing
down, so we're not aware of the motion. (In physics we tend to call it velocity rather than speed. Velocity is a vector
quantity having both magnitude and direction. The magnitude of the
vector is the speed). Instrumentation can't directly detect velocity,
except from relative motion to something else. So for instance in a car,
speed was traditionally detected mechanically by having a flexible
steel cable connected between the mechanical speedometer and
transmission, but more likely electronically nowadays using a Hall
Effect sensor or similar to count rotations of a component in the
transmission such as the front axle. (Like the sensor and magnet on your
bicycle wheel to measure speed). Aircraft airspeed can be measured
using tubes called pitot tubes
connected to the fuselage, which measure drop in air pressure as air
flows over the openings. (The same pressure drop principle used in
carburetors of petrol engines and paint sprayers). The variation in air
pressure then gets processed and ultimately results in a display of
airspeed on a gauge on the instrument panel.
What Happens if There's Nothing to Reference Against?
Navigation
is possible using the Sun, stars, a compass and a good timepiece.
However another way without resorting to astronomical observations or
knowing longitude is to use dead reckoning. So if you know the direction
you're travelling from a compass and measure the length of time for
which you were travelling and the average speed you were travelling at,
you can simply multiply speed by time to find the distance and angle
from the start point. This can be done repeatedly at intervals, using
compass readings and speed to calculate position and plot a trajectory
of a vehicle or other object. If speed isn't constant, an integration
process can be used to add up all the instantaneous speed x time values.
For a spacecraft or missile that doesn't have wheels or air around it
to measure velocity from, accelerometers have to be used. These measure
acceleration, and from acceleration, velocity and also distance
travelled can be calculated. (Mathematically, velocity is the integral
of acceleration and distance travelled is the integral of velocity).
Direction of travel is also measured in three dimensions by determining
the relative angle of a craft (attitude) with respect to references
provided by three gyroscopes. The complete system with gyroscopes,
accelerometers and associated components and electronics is known as an inertial guidance system.
