Is Saturn a classical or modern planet?

Answer 1

Depending on whether this is a question of astronomy or Astrology, the answer differs:

In astrology, the classical planets are those known to ancient astrologers, and used by classical astrology: The Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn. Prior to the more modern discovery of the outermost planets, astrology was based on these bodies as well as the stars. After the discovery of the outermost planets, Neptune, Uranus, Pluto, and Ceres (purportedly a lesser-known dwarf planet beyond Pluto's orbit), these too were incorporated into the body of astrological knowledge. Thus, the modern astrological planets (for the computation of astrological matters) would include all of these, while classical astrology would only consider the seven former 'planets.'

From the perspective of astronomy, aside from the strict number of planets (the outermost planets in the Earth's solar system were only discovered more recently), there is a fundamental difference in the way that our solar system's orbital mechanics are viewed today, as opposed to during classical times.

Galileo was the first to suggest a heliocentric (sun-centered) solar system. Prior to his discovery (and subsequent execution for uttering such a heresy), it was believed that the earth was the center of the universe. This is often called 'Copernican' theory, named after Copernicus, who first posited an Earth-centered universe. According to this hypothesis, the sun, stars and all the planets orbit the earth. Though today, every school child knows this to be false, strong evidence for this can be observed by simply watching the sun and stars -- any of which can be observed to move across the sky in such a path as to suggest that they orbit the earth. However, even during classical, times astronomers were at a loss to explain the strange orbits of the planets. The word 'planets' comes from the greek 'planetoi' -- wanderers -- an acknowledgement of this difficulty. Unlike stars and the sun, when tracked over time, the planets seem to follow quite erratic paths, which don't lend themselves to seasonal predictability, and don't simply move the same direction as the sun and stars. Prior to Galileo, astronomers invented myriad complicated orbital schemes and diagrams to explain how the planets orbited the earth. These diagrams usually consisted of sub-orbitals, in which the planets basically stall while transiting smaller eccentrics along their main orbits, slowing and speeding up, as observed from earth. This is what may be referred to as classical Copernican orbital mechanics.

Of course, all of this was completely false. Galileo claimed that all planets follow a more or less circular orbit, and eccentricities observed were explained by the fact that they were not in fact orbiting the earth, but the sun.

Answer 2

Classical. Planets that are visible to the naked eye are "classical" because they were known to the Greeks, Egyptians and Babylonians. They are Mercury, Venus, Mars, Jupiter and Saturn.

"Modern" planets are not visible without a telescope. These include Uranus and Neptune, and all of the dwarf planets, minor planets and asteroids.