Shifting Light

Have you ever pondered the movement of a star
 A planet or a galaxy up in the night sky
 Have you ever wondered why the sound of a siren
 Is constantly changing as it passes you by
 It's good to have many questions
 About the world that you face
 That's how all of human kind's
 Discoveries are made
 The sound of that siren and light of that star
 Both are ever changing, it depends on where you are
 In relation to the source of sound or light alike
 The frequency that you receive very well might
 Cause sound to become lower in pitch
 Or light to red-shift
 This change in observed frequency
 Found by Doppler, the physicist
 Just as the sounds of sirens change
 As they come and go
 The light from other galaxies
 Helps us come to know
 The universe is vast in size
 Growing all the time
 As shown by red-shifting light
 That's come to meet our eyes