What planets did ancient astronomers know about?
- Mercury
- Venus
- Mars
- Jupiter
- Saturn There’s something in common with all of these planets— they’re bright enough to see with the naked eye, which was the only tool that ancient astronomers had until the popularization of the telescope by Galileo in the seventeenth century.
Due to them performing every observation and measurement by hand, there was bound to be a significant margin of error.
What looked ‘weird’ to ancient astronomers?
Planets usually move slightly eastward from night to night, relative to the stars. Sometimes, though, they reversed the way they were moving and went westward relative to the stars for a period of time. This observation of ‘backwards movement’ is referred to as apparent retrograde motion.
Retrograde Motion
Retrograde motion appears to occur in our night sky when the ‘lap’ another planet around the Sun, or whenever a planet ‘laps’ us.
Knowing that the objects in the Solar System orbit around the Sun, we can make sense of apparent change in direction pretty easily. However, this was puzzling to people who though the Earth was the center of the universe, like many ancient figures did.
But ancient astronomers weren’t stupid. They did actually consider that the Sun could be the center of the universe, with some like Copernicus embracing it, but ultimately did not believe it to their inability to observe the stars in the sky actively moving throughout the year, a phenomenon dubbed stellar parallax.
The inability to observe stellar parallax leads to two paths of logical reasoning:
- The stars are so far away that stellar parallax is too small to notice with the naked eye.
- Earth does not orbit anything, it is the center of the universe. The ancient chose to believe the second option, perhaps due to cultural philosophies about perfection and them being the paragon of civilization. Or maybe they just liked it better, I don’t know, I wasn’t there y’know.