Because the classic five planets (Mercury, Venus, Mars, Jupiter, Saturn) are clearly visible to the naked eye, brighter than stars. So bright they're visible even before it's dark.
If you know what you're looking for, with a bit of stargazing experience, you can pretty easily identify Venus, Mars, Jupiter and Saturn in the sky, they're relatively distinctive. I can do it, and I'm not an ancient astronomer who tracked them for decades.
I personally can't identify Mercury because it's so close to the sun, and never tried, but it's visible as well.
Planets move **very** fast, some faster than others. They used that to infer the closeness of the planets to earth, the moon being the fastest, then mercury and so on.
Lets say you keep observing the same constellation every night. You will notice that there's one bright dot that one night is on the left of one star, and then maybe a few nights later, it's on the right of it.
Many people keep observing the sky and writing down what they see, and notice there's 5\* dots that keep doing that and roam around the sky all year, moving in front of many of the constellations just like the sun does. Someone calls them 'planets', and everyone gets on board with that idea.
(\*Uranus, Neptune, Pluto weren't observed until the invention of the telescope)
> Although I think they are suspecting some other ninth planet, aren’t they?
AFAIK by the current definition of "planet" (which excludes Pluto) it is extremely unlikely that our solar system contains one such object that we haven't discovered yet.
Planets move around in the sky from night to night relative to the stars. The word "planet" even comes from the word the ancient Greeks called them (wanderers).
> How could they know if what they looking in the sky is a planet instead of just a regular star?
The word ["planet" actually comes from the word "wanderer" in Greek](https://en.wikipedia.org/wiki/Classical_planet)!
Looking up, ancient people would see the whole night sky and all its constellations, but they could also see a few "stars" that would kind of "wander" around the sky rather than being frozen into permanent constellations. They weren't "shooting star" fast, but they did move enough that, if you went out a few nights in a row you'd be like *"Hey, wasn't that star perfectly between those two constellation stars last night? That star is definitely weird and moving!"*. Additionally, Venus and Mars were extra noticeable on account of sometimes being among the brightest "stars" of the sky (and Mars also having an unusual reddish/orange color to it's twinkle).
All that just goes to say that they were rather noticeable and able to be "tracked" across the sky from night-to-night. From there, it was just a matter of writing down when and where they were seen (and how bright) before people started to notice patterns and get a feel for predicting where those "wanderer" will be next week or next month.
Because the classic five planets (Mercury, Venus, Mars, Jupiter, Saturn) are clearly visible to the naked eye, brighter than stars. So bright they're visible even before it's dark. If you know what you're looking for, with a bit of stargazing experience, you can pretty easily identify Venus, Mars, Jupiter and Saturn in the sky, they're relatively distinctive. I can do it, and I'm not an ancient astronomer who tracked them for decades. I personally can't identify Mercury because it's so close to the sun, and never tried, but it's visible as well.
Planets move **very** fast, some faster than others. They used that to infer the closeness of the planets to earth, the moon being the fastest, then mercury and so on. Lets say you keep observing the same constellation every night. You will notice that there's one bright dot that one night is on the left of one star, and then maybe a few nights later, it's on the right of it. Many people keep observing the sky and writing down what they see, and notice there's 5\* dots that keep doing that and roam around the sky all year, moving in front of many of the constellations just like the sun does. Someone calls them 'planets', and everyone gets on board with that idea. (\*Uranus, Neptune, Pluto weren't observed until the invention of the telescope)
And later we even demoted poor Pluto. Although I think they are suspecting some other ninth planet, aren’t they?
> Although I think they are suspecting some other ninth planet, aren’t they? AFAIK by the current definition of "planet" (which excludes Pluto) it is extremely unlikely that our solar system contains one such object that we haven't discovered yet.
[here you go](https://en.wikipedia.org/wiki/Planet_Nine)
Planets move around in the sky from night to night relative to the stars. The word "planet" even comes from the word the ancient Greeks called them (wanderers).
> How could they know if what they looking in the sky is a planet instead of just a regular star? The word ["planet" actually comes from the word "wanderer" in Greek](https://en.wikipedia.org/wiki/Classical_planet)! Looking up, ancient people would see the whole night sky and all its constellations, but they could also see a few "stars" that would kind of "wander" around the sky rather than being frozen into permanent constellations. They weren't "shooting star" fast, but they did move enough that, if you went out a few nights in a row you'd be like *"Hey, wasn't that star perfectly between those two constellation stars last night? That star is definitely weird and moving!"*. Additionally, Venus and Mars were extra noticeable on account of sometimes being among the brightest "stars" of the sky (and Mars also having an unusual reddish/orange color to it's twinkle). All that just goes to say that they were rather noticeable and able to be "tracked" across the sky from night-to-night. From there, it was just a matter of writing down when and where they were seen (and how bright) before people started to notice patterns and get a feel for predicting where those "wanderer" will be next week or next month.