The Romans called the planet after Jupiter, their ruler of gods and deity of the sky and thunder. Jupiter was the biggest object in the sky, and hence the most powerful. So, it made sense for them to give our world's largest moon name after him. They also believed that Jupiter looked down on his people from the clouds every morning before dawn. This is why they called the first moon of Earth "Jupiter Moon" or "Luna Jovis". This is how the term "lunar" came to mean "relating to the moon": planets that orbit the sun, asteroids that orbit the earth, and comets that orbit the sun come under the category of lunar objects. Lunar means "moon-like". The moon is the only object in the solar system that fits this description. All other bodies either belong to a different planet or are part of a larger body (such as an asteroid cluster or a comet). Comets are fragments of a giant planet or star that has fallen into the gravitational field of the sun. They are carried with great speed through space by the solar wind. When they approach the sun, the heat causes them to evaporate, leaving a gaseous tail behind.