Planets were written about for about 2,500 years and have most likely been observed longer. They called them the wanderers for many centuries, as they appeared to glow like stars but moved throughout the sky. They also observed that stars were fixed in place, where as planets wandered around. Further, they could tell they had a steady light.

They were viewed as rising and setting, much like how we see our sun. In some situations they would observe that they got close together, but they did notice patterns with these wanderers. For planets like Mercury and Venus, they were noted as followers as they appeared to follow Earth. Mars and Jupiter were noted as true wanderers.

Source

If you wish, this is a short article written in 1720 that observes these stars.

Hey gang. Just a mod reminder to keep answers about history that is "how did astronomers from ancient civilisations differentiate planets from stars", not how you or astronomers today tell the difference. Do ensure that your answers are based on solid sources that actually address the question. Thanks!

hi! check out these earlier posts

• What is the earliest record of a person theorizing that stars are suns and planets?

• When did scientists recognize the planets in our solar system as (relatively) nearby planets, and not as other stars. How did they know that these celestial bodies were planets and not stars?

these ones may be of tangential interest

• When did people realize that planets were not just moving stars?

• When did humans become aware that they were on planet Earth, in a solar system, and part of the Universe?

It might also be worth x-posting to /r/AskScience (astronomy) for info on early astronomical theory, and /r/AskAnthropology for info from other cultures

Just to put things in perspective:

If you're like most people today you hardly ever get a chance to look at the night sky because you're going to be indoors most of the time. And when you do, it's going to be largely polluted by city lights, so much so that you can't even see the Milky Way in most places.

But if you spend most of your life outside, esp. at night, and have almost no light source besides the stars and moon, you get plenty of opportunities to look at them and notice patterns, such as constellations. That some of the brighter stars are moving while the rest is static is actually pretty obvious.

They weren't. People thought the planets were stars until the 1600s. As zwlegendary pointed out, the word planet itself comes from the term "wandering star."

In the middle ages, the standard cosmology held that the Earth was at the center of the universe, and there were nine rotating celestial spheres containing, in order, the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the "fixed stars," so called because they seem to move together as the "wandering stars" circle in front of them. This is called the Ptolemaic model, after the ancient astronomer Ptolemy.

Galileo changed everything with his telescope. Aside from advancing the sun-centered Copernican model, and discovering the moons of Jupiter and the rings of Saturn (which he thought were also moons), he also noticed that Venus goes through phases like the moon, depending on its position relative to the sun, thus proving that it and the other planets were solid bodies and not light-producing stars.