Astronomy is a natural science that deals with the study of celestial objects (such as moons, planets, stars, nebulae, and galaxies); the physics, chemistry, and evolution of such objects; and phenomena that originate outside the atmosphere of Earth (such as supernovae explosions, gamma ray bursts, and cosmic background radiation). A related but distinct subject, cosmology, is concerned with studying the universe as a whole.
In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, such as Stonehenge, early cultures assembled massive artifacts that possibly had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year.
Before tools such as the telescope were invented, early study of the stars had to be conducted from the only vantage points available, namely tall buildings and high ground using the naked eye. As civilizations developed, most notably in Mesopotamia, China, Egypt, Greece, India, and Central America, astronomical observatories were assembled, and ideas on the nature of the universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. The Earth was believed to be the center of the universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the universe, or the Ptolemaic system, named after Ptolemy.