As the sun sets in the western sky the atmosphere acts like a giant prism and splits the light from the sum into its spectrum of colours. It is light from the red end of the spectrum that hits Uluru and is reflected off it. Hence it will appear redder as the sun goes down. Usually in the last fifteen minutes or so. The changes are subtle and not as bright as you see in some photographs. The following sequence was taken at sunset on March 22nd.
