Rudolph the Red-Shifted Reindeer

 Thanks to Ruth for this wonderful card.

Rudoph's red nose is emitting waves that spread out like ripples on a still pond when you chuck a stone in. However, notice that the ripples are closer together at A than they are at B. On a pond, they would be spread the same in every direction. The explanation here is that Rudolph is moving from right to left whilst continuing to emit ripples of light. Hence the ripples bunch up in front of him (shorter wavelength) but stretch out behind (longer wavelength). This stretching of the wavelength behind is called red shift. Red has the longest wavelength and any increase in wavelength of light is a shift towards the red end of the spectrum.