North Atlantic
Ocean
as seen from Finland
Early Spring, 9th century
Under normal conditions, you can't see Greenland from Iceland -- it is below the horizon line. But what if there's a superior mirage? Superior mirages make objects appear above where they really are. So, a superior mirage could make an island (like Greenland) appear to hover above the horizon.
Voilá! Greenland is visible from Iceland!
Early travellers like Eric the Red were familiar with mirages - they are very common over the North Atlantic, especially in the late spring, which was also the best time of year to travel on the ocean. Early sailors knew that mirages were illusions, but they also knew the illusions could tell them something about the real world.