International Space Station now has live 24-hour HD cameras

We've come a long, long way since mankind took its first photograph of earth from space, in 1946 - and this week, NASA started live-streaming high-definition video footage right from the International Space Station.

The NASA HDEV channel on Ustream shows footage from the four cameras mounted to the ISS as part of the High Definition Earth Viewing "experiment", and was switched on at the start of this month. The cameras are housed in a temperature- and pressure-controlled box attached to the European Space Agency's Columbus module.

These cameras transmit video constantly to ground stations, which are pushed online as they are available.

Black screens mean the station is on the dark side of the planet.

Grey screens mean the camera is switching from one angle to another.

Because the ISS orbits the earth once every 92.91 minutes (at a speed of about 27,000 kilometres an hour), the feed does turn black from time to time as the cameras point at the night side of the earth. If that happens, don't worry - it'll come around again soon.

You can also check the current location of the ISS on http://iss.astroviewer.net/ to get an approximate idea of the day-night cycle.