The moon's orbit. The moon's orbit is not on the same plane as the earth's orbit around the sun. If it were, every time we had a new moon we would have a solar eclipse, and every time we had a full moon we would have a lunar eclipse. Instead, the moon travels in a track that goes well above and well below the earth. Still, on occasion it will travel in between the sun and the earth and in this case, there will be an eclipse.
Ganymede is larger than Mercury, which is the innermost--and smallest--major planet in our Solar System. The surface area of Ganymede is more than half that of the land area of Earth, and it provides scientists with a wealth of data concerning a great variety of surface features.
On occasion February will have no full moons and January and March will both have two. In this case, you have two calendar blue moons in the same year. However, there can be no more than one in a year's time or it would not be that rare of an event. Using the Farmer's Almanac description there can be a maximum of one in a year.