Learn how astronomers used parallax to more accurately determine the distance to Cygnus X-1, a stellar black hole, in this video from NOVA: Black Hole Apocalypse. People perceive depth using the principle of parallax—our two eyes see the world from slightly different vantage points. To calculate the distance to a celestial object, astronomers use Earth’s orbit to provide two vantage points. Astronomers measured the tiny parallax angle of Cygnus X-1 using an array of 10 radio telescopes across the world. They then determined that Cygnus X-1 is about 6,000 light-years away and calculated its mass to be about 15 solar masses. This resource is part of the NOVA: Black Hole Apocalypse Collection.