Artist’s impression of a star occultation by Titan

On 14 November 2003 Titan, Saturn’s biggest moon, passed in front of two stars, just seven and a half hours apart. The first occultation was visible just after midnight from the Indian Ocean and the southern half of Africa. When such occultation events take place, the light from the star is blocked out. Because Titan has a thick atmosphere, the light does not ‘turn off’ straight away. Instead, it drops gradually as the blankets of atmosphere slide in front of the star, as the light-curve drawn here shows. The way the light drops tells astronomers about the atmosphere of Titan.

In particular, Titan’s atmosphere acts like a lens, so at the very middle of the occultation, a bright flash occurs (indicated by the central peak in the light curve). If Titan’s atmosphere were a perfectly uniform layer, the central flash would be a pinprick of light, visible only at the very centre of the planet’s shadow.