Here's why the Doppler Effect only provides information about radial velocity:
* The Doppler Effect is based on the compression or stretching of waves: When a source of waves is moving towards the observer, the waves get compressed, resulting in a higher frequency (blueshift). Conversely, when the source is moving away, the waves get stretched, resulting in a lower frequency (redshift).
* The compression or stretching depends on the relative motion in the direction of the wave propagation: The Doppler Effect only affects the component of the velocity that is directed towards or away from the observer – the radial velocity. Motion perpendicular to this direction (the tangential velocity) doesn't change the wavelength of the wave and therefore doesn't contribute to the Doppler shift.
Analogy: Imagine a boat moving across a still lake. If the boat moves directly towards you, you will hear the sound of the boat's engine at a higher pitch. If the boat moves directly away from you, you'll hear the sound at a lower pitch. However, if the boat moves parallel to you, the pitch of the engine sound doesn't change significantly because the relative motion is perpendicular to the direction of the sound waves.
In summary:
* The Doppler Effect measures the change in frequency of a wave due to relative motion.
* This change in frequency can be used to infer the radial velocity.
* The Doppler Effect doesn't provide information about the tangential velocity.
It's important to note that while the Doppler Effect only measures radial velocity, it is still a powerful tool for understanding the motion of stars, galaxies, and other celestial objects. By combining Doppler measurements with other observations, astronomers can obtain a more complete understanding of their motion in space.
