Learn Hardcore Italian: L’astronomia ci insegna che ogni pianeta segue la propria orbita. - Astronomy teaches us that every planet follows its own orbit.

In Italian, when the definite article la comes before a word starting with a vowel (here astronomia), you drop the a of la and replace it with an apostrophe. So la astronomia becomes l’astronomia. This process is called elision.

What is the function of ci in ci insegna?

The verb insegnare (“to teach”) in Italian takes an indirect object introduced by a (“to”). Instead of saying insegnare qualcosa a noi (“to teach something to us”), you replace a noi with the clitic pronoun ci, meaning to us. So l’astronomia ci insegna = “astronomy teaches us.”

What does che mean here and why is it necessary?

Here che is a subordinating conjunction meaning that. It connects the main clause (L’astronomia ci insegna) with the subordinate clause (ogni pianeta segue la propria orbita). Without che, the sentence would be ungrammatical.

Why is it ogni pianeta instead of i pianeti? Why singular and no article?

The word ogni (“each/every”) always takes a singular noun and does not need another article. You say ogni pianeta (“each planet”) rather than ogni i pianeti. If you wanted “all the planets,” you would use tutti i pianeti instead.

Could we say tutti i pianeti seguono le loro orbite? How would that differ?

Yes. Tutti i pianeti seguono le loro orbite literally means “all the planets follow their orbits.”
Difference in nuance:

Ogni pianeta segue la propria orbita focuses on each planet individually.
Tutti i pianeti seguono le loro orbite speaks of the group as a whole.
Also note the plural agreement: tutti (plural), i pianeti, loro orbite.

Why is it propria and not proprio?

Propria is a possessive adjective that must agree in gender and number with the noun it modifies. Orbita is feminine singular, so you use propria. If the noun were masculine or plural, you’d adjust: e.g., il suo percorso (masculine), le loro orbite (feminine plural).

Could we move propria after orbita, as in l’orbita propria?

Yes, you can: ogni pianeta segue l’orbita propria is grammatically correct. Placing the adjective after the noun is more neutral; placing it before (as in la propria orbita) can add slight emphasis on possession or formality.

Why is orbita feminine? It ends in -a but doesn’t refer to a female.

In Italian, noun gender is largely arbitrary and must be learned with the word. Many nouns ending in -a are feminine (e.g., casa, strada, orbita), but some (like problema, programma) are masculine. You memorize each word’s gender.

Why doesn’t seguire require a preposition before la propria orbita?

Seguire is a transitive verb when it means “to follow.” Transitive verbs take a direct object without a preposition. Here la propria orbita is that direct object: “(each planet) follows (its own orbit).”