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| Smile! Nasa to attempt long-distance portrait of Earth from Saturn | Smile! Nasa to attempt long-distance portrait of Earth from Saturn |
| (about 20 hours later) | |
| It may sound like a terrible sequel to The Day the Earth Stood Still, but The Day the Earth Smiled is an attempt to take a new picture of our home planet from a spacecraft in orbit around Saturn. | It may sound like a terrible sequel to The Day the Earth Stood Still, but The Day the Earth Smiled is an attempt to take a new picture of our home planet from a spacecraft in orbit around Saturn. |
| The project has been masterminded by Carolyn Porco, who leads the imaging team on Nasa's Cassini mission to Saturn. | The project has been masterminded by Carolyn Porco, who leads the imaging team on Nasa's Cassini mission to Saturn. |
| Cassini will acquire a sequence of images of Saturn and its ring system over the course of four hours on Friday. The Earth will be visible near the rings and will be captured as it appears between 22.27 and 22.42 BST (17.27 to 17.42 EDT). | Cassini will acquire a sequence of images of Saturn and its ring system over the course of four hours on Friday. The Earth will be visible near the rings and will be captured as it appears between 22.27 and 22.42 BST (17.27 to 17.42 EDT). |
| I say "as it appears" because Cassini is 1.44bn kilometres away. Light from Earth takes 80 minutes to reach it. So, although the time to look up and smile is between 22.27 and 22.42 BST, the shutters on Cassini will open 80 minutes later to capture the light as it arrives. Astronomers call this "look-back time". | I say "as it appears" because Cassini is 1.44bn kilometres away. Light from Earth takes 80 minutes to reach it. So, although the time to look up and smile is between 22.27 and 22.42 BST, the shutters on Cassini will open 80 minutes later to capture the light as it arrives. Astronomers call this "look-back time". |
| In the finished image, our entire planet will appear as no more than a pale, blue dot, peeping through Saturn rings. | In the finished image, our entire planet will appear as no more than a pale, blue dot, peeping through Saturn rings. |
| "Pale blue dot" was the phrase used by Carl Sagan to describe an image of Earth taken in 1990 from even further away. The Voyager 1 spacecraft was 6bn kilometres from us, roughly the distance to Pluto's orbit, when it took pictures of most of the planets in the solar system. | "Pale blue dot" was the phrase used by Carl Sagan to describe an image of Earth taken in 1990 from even further away. The Voyager 1 spacecraft was 6bn kilometres from us, roughly the distance to Pluto's orbit, when it took pictures of most of the planets in the solar system. |
| Because of Earth's predominant oceans, the planet appeared pale blue but was difficult to pick out on the final image. | Because of Earth's predominant oceans, the planet appeared pale blue but was difficult to pick out on the final image. |
| It is hoped that Cassini's image will be more photogenic. The picture will be released soon after the event. | It is hoped that Cassini's image will be more photogenic. The picture will be released soon after the event. |
| "Waving time" for various time zones around the world can be found here, although not all of the continents will be in view. Only North America and part of the Atlantic Ocean will be facing Saturn at the time of the image. | "Waving time" for various time zones around the world can be found here, although not all of the continents will be in view. Only North America and part of the Atlantic Ocean will be facing Saturn at the time of the image. |
| Stuart Clark is the author of Voyager: 101 Wonders Between Earth and the Edge of the Cosmos (Atlantic). Find him on Twitter as @DrStuClark | Stuart Clark is the author of Voyager: 101 Wonders Between Earth and the Edge of the Cosmos (Atlantic). Find him on Twitter as @DrStuClark |
| • This article was amended on 20 July 2013. The standfirst originally stated that the Earth's population is six billion. This has been corrected. |