Sidus erraticum inter stellas movetur.

Sidus is a fairly broad word for a heavenly body, constellation, or celestial phenomenon. In this sentence, because it is modified by erraticum meaning wandering, sidus erraticum refers to a wandering heavenly body, that is, a planet.

Stella is the more ordinary word for a star. So the sentence contrasts the moving sidus erraticum with the fixed stellas around it.

Because erraticum is an adjective agreeing with sidus.

Latin adjectives must match the nouns they describe in:

• gender
• number
• case

Here, sidus is:

• neuter
• singular
• nominative

So the adjective must also be:

• neuter
• singular
• nominative

That gives erraticum.

If the noun were masculine, you would expect erraticus; if feminine, erratica.

Sidus is nominative singular, and it is the subject of movetur.

You can tell because:

• the verb movetur is third person singular
• sidus is singular and fits as the thing doing or undergoing the action
• stellas cannot be the subject, because it is in the accusative plural after the preposition inter

So the basic structure is:

sidus erraticum = subject
movetur = verb
inter stellas = prepositional phrase

Because inter takes the accusative case.

So:

• stella = star
• stellae = stars, nominative plural
• stellas = stars, accusative plural

After inter, Latin uses the accusative, so inter stellas is the correct form.

This is something English speakers just have to learn as part of the preposition:
inter + accusative.

It can mean either, depending on context.

With a plural noun like stellas, English often prefers among the stars, because between can sound as if there are only two. But Latin inter works perfectly well with plurals and does not force that distinction in the same way English does.

So in this sentence, among the stars is often the most natural English phrasing, though between the stars is also possible.

Movet is the active form and usually means moves in the sense of moves something.

For example:

• ventus ramum movet = the wind moves the branch

Movetur is the passive form and literally means is moved. In a sentence like this, English often translates it more naturally as moves or is moving, because the point is that the heavenly body is in motion, not that it is moving some other object.

So:

• movet = moves something
• movetur = is moved, is in motion, moves

Movetur tells you several things at once:

• present tense
• third person
• singular
• passive voice
• from the verb moveo, movere

So it means he/she/it is moved or, in smoother English here, it moves or it is moving.

Since the subject is sidus, the sense is the wandering body moves / is moving.

Because Latin word order is much more flexible than English word order.

In English, we normally say wandering star with the adjective first. In Latin, the adjective can come before or after the noun:

• erraticum sidus
• sidus erraticum

Both are possible.

Latin relies more on endings than on position to show how words relate to each other. So putting erraticum after sidus is perfectly normal.

Yes. Latin word order is flexible, so several arrangements are possible, such as:

• Sidus erraticum inter stellas movetur
• Inter stellas sidus erraticum movetur
• Movetur inter stellas sidus erraticum

These all contain the same basic grammar. The choice of order can affect emphasis, style, or rhythm more than core meaning.

The version you have is a very straightforward and natural order:

• subject first
• then a prepositional phrase
• then the verb

Because Latin has no articles.

So sidus can mean:

• a star
• the star
• simply star in a general sense

And stellas can mean:

• stars
• the stars

The context tells you which English article is best. This is one of the biggest differences from English, where articles are used constantly.