Mars’ moon Phobos is unlike Earth’s Moon in most ways. For starters, it zips around Mars three times a day. Phobos practically hugs its host — orbiting just 3,728 miles (6,000 kilometers) away. Our Moon averages 238,900 miles (384,402 kilometers) of distance. From Mars, Phobos would appear about one-third as big as Earth’s Moon. If you stood on Phobos, Mars would fill almost the entire sky, astronomers say.