Eclipse Shadow bands on Cloud ~
The web is full of images of the March 9, 2016 total solar eclipse over Indonesia. But here, Muhammad Rayhan of the Jakarta Planetarium & Observatory has captured something rare. Look below the black sun. The cloud is covered with parallel diagonal bands. These are the fabled 'shadow bands' sometimes seen flickering over the ground at the beginning and end of totality. Here the bands are projected not on the ground but on a translucent layer of intervening cloud. To complete the scene, the narrow slit of light shining along the lunar rim has produced some iridescence and even an elongated (atmospheric) corona in the cloud droplets.
Image ©Muhammad Rayhan
Paired caustic sheets from an idealised wavy boundary of two different density air layers high in the sky.
Wavy temperature and density irregularities high in atmosphere produce the shadow bands. The slit-like sources of light at the beginning and end of totality are needed to show them up.
The background image shows an idealised wave between a lower denser layer and an upper more rarefied one.
Computer traced parallel rays head downwards and refract at the wavy boundary. They are not ‘focussed’ in the way that a lens acts. Instead they form caustics, sheets of intense light with surrounding dark areas.
The bright sheets and intervening darkness shows up where they are cut by a surface – in this case a thin cloud layer – that acts as a screen. Not all atmosphere irregularities are strong enough to form caustics but they are frequently present much to the chagrin of astronomers.