within the solar system, there is a connection between the period of an orbit and the radius of the orbit
A specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity
It is the change in frequency (and wavelength) of light, caused by the relative motion of the source and the observer
‘Red shift’ is a key concept for astronomers. The term can be understood literally – the wavelength of the light is stretched, so the light is seen as ‘shifted’ towards the red part of the spectrum. Something similar happens to sound waves when a source of sound moves relative to an observer.