Ancient astronomers were able to differentiate between stars and planets, as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time.

The real renaissance of astronomy began with Nicholaus Copernicus, who advanced the idea that the Sun is in the center of the Solar System. Armed with the excellent naked-eye observations of Tycho Brahe,  Johannes Kepler formulated his Three Laws of Planetary Motion, which, for the first time, correctly described the way the planets move through the Solar System. Galileo Galilei was the first person to use a telescope to look at celestial bodies (though he did not invent the telescope) and discovered the four brightest moons of Jupiter, proving that there are things in the Solar System that don't revolve around the Sun.

Since Galileo's time, astronomy has made great strides, but, surprisingly, as late as the 1920's, astronomers were still debating about whether other galaxies were simply nearby nebulae, or if they were faraway "island universes," made up of billions of stars. The first planets outside our solar system were not discovered until 1991, and we did not find any planets around normal stars until 1995.