Earth, Moon, and Our Solar System Vocabulary Rotation means to spin around an axis. Revolution means to travel in a closed path. An axis is an imaginary line that runs through the center to the Earth from the North Pole to the South Pole. A phase is one of the shapes the moon seems to have as it orbits Earth. Orbit is a closed path of one object in space around another object, or to move in such a path. Seasons are caused by the tilt of the Earth. The Earth is divided into two spheres called hemispheres. These are the Northern and Southern hemisphere. A star and all the planets and other objects that revolve around it is called a solar system. A planet is a large body that revolves around a star. The galaxy is a huge system of gases, dust, and many stars. A star is a huge ball of superheated gases. A pattern of stars that form an imaginary picture or design in the sky is called a constellation. Optical Telescopes are used to study faraway objects in space. Cyclical means to recur in cycles (a series of events that regularly repeat in the same order). Crewed or "manned" missions have humans aboard to control the spacecraft and handle its operations. The sun is the star at the center of our solar Space probes are longterm satellites that have no one aboard. A satellite is an artificial body placed in orbit around the earth or another planet in order to collect information or for communication. Reflection is the bouncing of light off of an object.