The surface of Mercury, as photographed by the Mariner 10 craft, circa 1974. (Photo by Space Frontiers/Hulton Archive/Getty Images) On Sunday, the planet Mercury will be at the furthest distance from the sun — known as its greatest elongation — during its October-November morning apparition, according to In-The-Sky.org.The website defines an object’s apparition as "a period of weeks or months during which it is visible in the night sky."They also said because Mercury is so close to the sun, it’s often lost in the sun’s glare.