Some day an astronaut will stand on Mars and look back at Earth. As Alfred, Lord Tennyson wrote Venus, Hesper, Were we native to that splendour or in Mars We would see the globe we groan in, fairest of their evening stars Could we dream of wars and carnage, craft and madness lust and spite Roaring London, raving Paris, in that peaceful point of light? Would we not, when gazing heavenward, at a star so silver-fair Yearn, and clasp the hands, and murmur: Would to God that we were there? But what exactly will one see from Mars? An observer on Earth needs a powerful telescope to see the tiny moons of Mars, Phobos and Deimos, but our own moon is much larger, and orbits at a greater distance. Could the unaided eye of an observer on Mars tell apart the Earth and its moon, at their greatest separation? One might ask a similar question about the four big moons of Jupiter, discovered by Galileo. They are about as big as our own moon. Could an observer on Earth distinguish any of them from their planet?