Eli5: How could humans tell the difference between a planet and a star in the nightsky, before telescopes where invented?
In: Planetary Science
Planets were called “wandering stars” by the Greeks, who were among the cultures that noticed them move a little bit between the motionless stars night after night.
If you have for example Leo, and you notice a bright red star in it, a week later you can see it has changed positions compared to everything else in the layout of the stars, and you’re gonna consider it a different type of star altogether.
Also, planets don’t twinkle… not nearly as much as stars do, anyway.
They didn’t.
“Planet” literally means Wandering Star. To ancient people, the planets as we know them were literally thought of as just regular stars that just happened to move around a lot in relation to other stars.
They were given names to differentiate them.
Stars follow a regular path through the sky, whereas planets appear to make loops.
The word ‘planet’ comes from the Greek ‘planan’ (to wander) because of this movement.
When they thought the earth was at the centre of the universe, [they had to invent complex paths for the planets to explain their movement](https://i.imgur.com/FlkiyQw.jpg).
Of course, once you realise that the sun is at the centre of the solar system, and that the earth is one of several planets going around it, the patterns seem a lot simpler!
Additionally, planets don’t twinkle the way stars do. This is because their angular diameter is not a point like stars are. Also planets like Mars and Jupiter have an obvious colour. Especially Venus is much brighter than any other stars.
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