As applied to waves of electromagnetic radiation, phase is the relative measure of the alignment between two waveforms of similar frequency. They are said to be in phase if the peaks and troughs of the two waves match up with each other in time. They are said to be out of phase to the extent that they do not match up. Phase is expressed in degrees from 0 to 360.
The phase difference between two waves, or phase change in a single wave, can apply in different areas of interplanetary space flight. Phase shifts in a spacecraft's telecommunications radio signal can be employed to carry information. Observations of effects from the Sun (or other body) upon the phase (and other characteristics) of a radio signal can provide information about that body to radio scientists. Interactions between radio waves, or light waves, given their phase relationships, can also be useful in many ways.
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