An American satellite obtained a unique view of a lunar eclipse

The DSCOVR spacecraft, which was launched in February, has a camera that stares constantly at the sunlit face of the Earth. The images are being used to track moving features such as clouds and dust storms, and to monitor the climate. But on 27 September, it was in just the right position to see the Moon go behind the Earth and into its shadow. On the ground at this time, skywatchers would have observed the lunar body turn a shade of red. It does this because some sunlight is still able to reach the Moon's surface after being filtered through the Earth's atmosphere. "Our camera is normally centred on the Earth but we use the Moon for calibration," explained Jay Herman, the US space agency's (Nasa) lead investigator onDSCOVR's Epic camera system. "That's what we were doing on this occasion. We were staring at the Moon and the Earth moved in front about four hours before the eclipse was seen on Earth. And that's because we were at an angular position, just to the side of the Sun-Earth line. "The Earth is rotating as it goes by. It's kind of neat because you can watch the motion of the clouds."

Dr Herman was speaking at the Fall Meeting of the American Geophysical Union in San Francisco - the largest annual gathering of Earth scientists.The DSCOVR satellite is reported to be in excellent shape. In-space testing continues following the launch but science operations have started up nonetheless, and the AGU gathering is being used to report some early observations and results. One of the satellite's objectives is to track cloud behaviour. The different wavelength filters on Epic allow the camera to estimate cloud heights. This is important not just for monitoring weather systems, but also to understand the clouds' impact on the climate. Some help to cool the planet by reflecting sunlight back out into space; other clouds actually warm the Earth because they trap heat.

And in pursuing this work, Epic data has already observed some unexpected things, like the tracks of ships. These are not the wakes created by the vessels cutting through the ocean surface, but rather the clouds their exhaust system are seeding up above. "It was very surprising for us that we could see them from one million miles, and they're even better seen if we use a longer wavelength because this gives you a strong contrast with the dark ocean," said Alexander Marshak, the DSCOVR deputy project scientist.Also still being tested is the satellite's NISTAR instrument. This is a radiometer that measures the total amount of solar energy being reflected off Earth, as well as the heat emitted from our planet.

Its principal investigator is Steven Lorentz from the US National Institute of Standards and Technology. He explained how the amount of solar energy bouncing off the planet would vary depending on which continents or oceans are in view. The Earth was most reflective, he said, when Africa dominated the scene (land surfaces are brighter than sea surfaces) with Antarctica visible also during the austral summer. "When the Earth is tilted this way or that way, it really does make a difference to the planetary albedo. And that just underlines the ice's importance to the climate because if the poles weren't there, or as they shrink, the amount of energy going into the system will go up."DSCOVR's project scientist, Adam Szabo, added: "The type of measurements that the satellite is making is nothing new - we've been making these exact observations from low-Earth orbiting or geostationary satellites. But the vantage point is really unique.

"Sitting between the Sun and Earth, we are seeing the full sunlit face of Earth all the time, allowing Earth to rotate beneath us rather than the satellite going around the Earth."