The astronomical knowledge of ancient cultures is the foundation of modern astronomy, including the idea of dividing the sky into groups of stars, each of which is called a (1)constellation , which they imagined traced out pictures in the sky. Ancient astronomers believed that all stars were fixed on a sphere surrounding the Earth, called the (2)celestial sphere. While this is not true, astronomers today find this model helpful in visualizing the (3) of celestial objects. Some points and lines on the celestial sphere are particularly useful; the point directly above the head of an observer is called the (4) , and the sky seems to turn about two points known as the
(5) . The apparent path followed by the Sun around the celestial sphere is called the (6) .
Ancient astronomers invented models of the universe to explain the apparent motions of visible objects in the heavens. Greek scholars believed the (7) is stationary and located at the center of the universe. A model of this kind is called a (8) model. Later, Greek astronomer (9) expanded this model with each planet moving in a small circle called an (10) , whose center in turn moves following the rotation of a larger sphere called the deferent.
Today we know the Sun only appears to rise and set each day. This is not due to an actual motion but rather due to Earth´s (11) . Over the course of the year, the constellations visible in the night sky slowly change. This is due to the (12) motion of the (13) around the (14) . It was until the sixteenth century that Polish astronomer
(15) devised the first comprehensive (16) model, with all the planets moving on circular orbits around the (17) . Johannes Kepler refined this model based on the astronomical observations of Danish astronomer (18) , and developed a series of principles known as Kepler’s three laws, which describe the planetary orbits as (19) , with the sun located at one (20) .