The definition of a planet has changed over the course of millennia, and during the last 100 years. In the 1820s, astronomers listed eleven planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus (Neptune had not been discovered yet), plus Ceres, Vesta, Juno, and Pallas.
The definition of a planet has changed over the course of numerous, and during the last 100 years. At some point in the Stone Age - it is impossible to know when or where- people noticed that a few of the points of light in the night sky moved with respect to the numerous others. Planets moving in relation to the stars is very difficult to detect in the course of a few hours in a single evening. From night to night, however, the movement of a planet can be very obvious, particularly for the inner planets: Mercury, Venus, and Mars. Planets, as well of the moon, appear in the sky in a band of constellations known as the Zodiac. Because the inclination of the moon’s orbit around Earth is only 5.5 degrees with respect to Earth’s equator, and because Earth and other planets orbit inclined within a few degrees of one another, and inclined within several degrees of the Sun’s equator, the Zodiac is a band running several degrees on either side of the ecliptic, the path that the Sun appears to take through the sky. Thus, from medium and high latitudes of the Northern Hemisphere, the Zodiac constellations and the planets and Moon appear to the south, while they appear to the north from the Southern Hemisphere.
The planets Uranus and Neptune would remain unknown to humanity until discovered the years 1781 CE and 1846 CE, respectively, but the movements of Venus, Jupiter, Mars, Mercury, and Saturn all were observed in detail throughout ancient times; not only are these planets visible to the naked eye, Venus and Jupiter are strikingly bright. So conspicuous for their movement in relation to what came to be known as the "fixed stars", these five planets were awarded special status in ancient civilizations throughout the world. Together with the Sun and Moon, they constituted seven wanderers through Earth's skies. And since each of these bodies was thought to be -or to be controlled by- a god or goddess, the number seven was considered sacred by cultures around the world. This is the reason for developments such as the seven day week, and the seven branched menorah and Sabbath in ancient Israel. There, the third brightest object in the sky (after the Sun and Moon), which the Romans equated with the goddess Venus, was associated with the Hebrew fertility goddess, Asherah, just as the Greeks associated it with Aphrodite. Although Judaism came to be dominated by monotheistic priests, and later rabbis, who still deny Israel's service to seven sky deities in ancient times, such a history is fairly obvious, given the emphasis that Israelite and Judahite writings and iconography placed on the number seven -not to mention the Bible's notation that King Solomon worshipped celestial bodies at the temple whose foundation traditionally is attributed to him.
For countless ages, even the most practiced astronomers assumed that all objects in the sky revolved around Earth. It was not until the 3rd century BCE that Ionian Greek, Aristarchus of Samos, proposed the heliocentric model. In addition to proposing that the Earth moved around the Sun, he proposed that the Sun was six to seven times wider than Earth, which is to say hundreds of times larger in volume. Aristarchus’ writings were among the works of the ancient world lost when the Library of Alexandria was burned during the 4th century CE Information known about Aristarchus comes from writings of some of his contemporaries, particularly Archimedes. Based on these writings, some scholars believe that Aristarchus first concluded that the Sun was much larger than Earth, then based on this idea reasoned that it made no sense for a larger body to orbit a smaller body. For many centuries, Aristarchus' heliocentric model was ignored and even forgotten, until revived by Copernicus, in the 16th century CE.
The advent of telescope-based astronomy by Galileo led not only to the discovery of the two large planets Uranus and Neptune, but also to a great deal of rethinking about what should be called a planet. For about 50 years, Ceres, which orbits the Sun between the orbits of Mars and Jupiter (thus in the asteroid belt), was called a planet. Later, it was called an asteroid, then finally it was classified as a dwarf planet; unlike an asteroid, Ceres is massive enough so that its gravity has pulled into a spherical shape. Similarly, Pluto was called a planet from 1930-2006, though it too now is classified among the dwarf planets, which though spherical are not massive enough to clear their orbits around the Sun of other bodies. For those who may be upset about Pluto's demotion, keep in mind that were Pluto to be called a planet, several other bodies would have to be called planets as well. In the 1820s, astronomers listed eleven planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus (Neptune had not been discovered yet), plus Ceres, Vesta, Juno, and Pallas. Various celestial bodies have been discovered, around the size of Pluto, or larger, as in the case of the dwarf planet, Eris. Today, all of these would have had to be called planets, if Pluto were called a planet, since they are massive enough to be spherical. Like Pluto, however, they are not large enough to clear their path of other objects. Currently, astronomers classify five objects as dwarf planets: Ceres, Haumea, Makemake, Eris, and Pluto. It is expected that this number will grow as more celestial bodies are studied in more detail, so that at some point, the Solar System will include seven dwarfs.