This view of the twilight sky and Martian horizon taken by NASA's Curiosity Mars rover includes Earth as the brightest point of light in the night sky. Earth is a little left of center in the image, and our moon is just below Earth.
Researchers used the left eye camera of Curiosity's Mast Camera (Mastcam) to capture this scene about 80 minutes after sunset on the 529th Martian day, or sol, of the rover's work on Mars (Jan. 31, 2014). The image has been processed to remove effects of cosmic rays.
A human observer with normal vision, if standing on Mars, could easily see Earth and the moon as two distinct, bright "evening stars."
The distance between Earth and Mars when Curiosity took the photo was about 99 million miles (160 million kilometers).
Image Credit: NASA/JPL-Caltech/MSSS/TAMU
Explanation from: http://photojournal.jpl.nasa.gov/catalog/PIA17936