<Tr> <Td> ← Previous "When Knowledge Conquered Fear" </Td> <Td> Next → "Hiding in the Light" </Td> </Tr> <Tr> <Td_colspan="2"> List of Cosmos: A Spacetime Odyssey episodes </Td> </Tr> <P> "A Sky Full of Ghosts" is the fourth episode of the American documentary television series Cosmos: A Spacetime Odyssey . It premiered on March 30, 2014 on Fox and on March 31, 2014 on National Geographic Channel . The episode presented an in - depth treatment of black holes, beginning with John Michell's suggestion of the existence of an "invisible star" to the first discovery of a black hole, Cygnus X-1 . The episode's title is an allusion to how light from stars and other cosmic objects takes eons to travel to Earth, giving rise to the possibility that we might be viewing objects that no longer exist . </P> <P> The episode received positive reviews, with one critic remarking on how the "high level of production value is consistently entertaining and informative ." Despite favorable reviews, however, the episode received a 1.5 / 4 in the 18 - 49 rating / share demo, with 3.91 million Americans watching it live on Fox, showing a steady decline from the series premiere . </P>

Cosmos episode 4 a sky full of ghosts