Since the ancients didn’t have a telescope to look at stars and just noticed some that wandered (thus the term planet), who was the first person, and when, to find out that they weren’t actually suns that were wandering but peices of rock like our own? And how did they figure it out?
I don’t know if anyone earlier figured out the nature of planets through inference, but Galileo was one of the first to observe planets through a powerful enough telescope to make out the actual discs (as opposed to points of light).
Depends what you mean by “weren’t stars”. Not only would people have to have inferred the nature of planets, they would also have to have inferred the nature of stars. I think Galileo first noticed that the wandering stars appeared to have a shape whereas the non-wandering stars never looked different from points of light. Is that enough?
All the ancient astronomers knew the difference between planets and stars. The Babylonians, the first people we have good astronomical records from, called them “wild goats” as opposed to the “tame goats” of the stars.
The Venus Tablets Of Ammizaduga date from the 8th century BCE but are thought to be copies of earlier tables from the 17th century BCE, or 3000 years earlier than Galileo.
Most of the Mediterranean peoples identified the wandering stars - the five visible planets plus the sun and the moon - with particular gods to emphasize their difference and importance. None of them thought that planets were like stars, but then they didn’t think our sun was like the stars either.
The actual nature of these bodies were the subject of numerous conjectures over the years. Galileo did more to suggest that they were earth-like by noticing that Jupiter had moons revolving around it than by seeing the planets as disks.
Mercury!
Venus!
Earth and Mars!
Jupiter, Saturn, Uranus
Neptune and Pluto!
The wandering stars
From a record I had when I was a kid. (A Children’s [something] Of The Universe or something like that.)
There is no factual answer to your question. The earliest proven observation of a planet is in the Venus Tablet of Ammisaduqa. They may have been made as early as the 20th century BCE, although the only copy we have dates from the 7th century BCE.
It would not have been difficult for the ancients to notice that all of the stars “move” in a circle except for a few that seem to roam randomly around the sky. Venus, being very bright, would be especially noticeable.
What the OP seems to be after is something like “when did someone realize that Venus was conclusively not simply a special kind of star?”
I think for what the OP seems to be after, direct astronomical observation with sufficiently powerful telescopes was the only way to figure out the difference. Galileo observed the sun in his telescopes, as well … and I assume that he realized that Venus, Mars, and Jupiter were reflecting the sun’s light while the sun itself was radiating light. Support for my assumption is that he was the first to observe the phases of Venus (analogous to moon phases), which is recounted in my cite in post #2 above.
What I don’t know is whether or not Galileo realized that all the observable stars were self-radiant objects much like our sun (diffferent sizes, ages, and temperatures, of course … but that information was a few centuries away).
Well, another thing is that it wasn’t clear until very recently that the stars were suns, or that the earth was a planet. I believe I recall that some ancient Greeks speculated that the stars were like our sun, just very far away. But that was speculation. The most common idea was that stars were something very different from the sun, and that the stars were some sort of feature of a vast sphere or shell that surrounded the Earth, with the Sun, Moon and planets as other sorts of things that moved around inside the star sphere.
The heliocentric theory was the key advance in highlighting the difference between planets and stars. Under the heliocentric theory, the planets (and the Sun) need to be several orders of magnitude closer to the Earth than are the stars (else the stars would show parallax).
Given that the stars are much farther away (in a heliocentric universe), they must also be vastly larger and brighter than the planets (to have the same apparent brightness).
Galileo’s observations thus served as a double whammy in establishing the Earth-like nature of the planets–first, by confirming heliocentricism, and second, by direct observation. Among other things, he observed the phases of Venus–direct and obvious proof that Venus wasn’t self-luminous.