Zodiacus circulus caelestis est.

Latin normally has no articles. It does not have separate words that mean the or a/an the way English does.

So Zodiacus circulus caelestis est can be understood as:

• The zodiac is a celestial circle
• A zodiac is a celestial circle

In most contexts, the natural English translation would be The zodiac is a celestial circle.

Est means is. It is the 3rd person singular present form of esse, meaning to be.

So:

• est = he/she/it is

Latin word order is much more flexible than English word order. A verb often comes at or near the end of the sentence, so ... est is very normal Latin style.

Both zodiacus and circulus are in the nominative singular.

That is because this is a sentence with to be:

• Zodiacus = the subject
• circulus caelestis = a predicate noun phrase describing the subject

In Latin, with esse (to be), both the subject and the predicate noun are commonly in the nominative.

So the structure is basically:

• Zodiacus = the zodiac
• circulus caelestis = a celestial circle
• est = is

Caelestis is an adjective meaning heavenly or celestial.

It agrees with circulus in:

• case: nominative
• number: singular
• gender: masculine

But not all Latin adjectives use endings like -us, -a, -um.
Caelestis belongs to a different adjective pattern, often called a third-declension adjective.

Its masculine and feminine nominative singular form is:

• caelestis

Its neuter nominative singular form is:

• caeleste

Since circulus is masculine singular nominative, caelestis is the correct form.

Yes, it can come before it.

Latin adjectives can come before or after the nouns they describe. So both of these are possible:

• circulus caelestis
• caelestis circulus

Both mean celestial circle.

The choice often depends on style, emphasis, or idiom rather than strict grammar. In this sentence, circulus caelestis is perfectly normal.

It is a Latinized Greek word. Many learned or astronomical terms in Latin come from Greek.

So zodiacus reflects Greek origin, but in the sentence it is being used as a regular Latin noun.

A learner does not need to know all the historical details to read the sentence correctly; the important thing is that here Zodiacus functions as a nominative singular noun.

Not with esse (to be).

In English, we say:

• The zodiac is a circle

Latin treats this as an equation or identification. Because of that, the complement after est is usually nominative, not accusative.

So:

• Zodiacus = nominative subject
• circulus = nominative predicate noun

If the verb were an action verb, then you might expect an accusative object. But est does not take a direct object here.

Yes. Because Latin uses endings to show grammar, the word order is flexible.

For example, these all mean roughly the same thing:

• Zodiacus circulus caelestis est
• Zodiacus est circulus caelestis
• Circulus caelestis zodiacus est

However, some orders sound more natural than others, and changing the order can shift emphasis.

The given sentence is a straightforward literary-style order:

• subject first
• description next
• verb last

From both sense and structure.

Grammatically, caelestis is masculine nominative singular, so it could in principle agree with either zodiacus or circulus, since both are masculine nominative singular.

But in the phrase circulus caelestis, the adjective is placed directly next to circulus, which strongly suggests that they belong together as a unit: celestial circle.

Also, the meaning fits naturally:

• The zodiac is a celestial circle

That is more natural than taking it as something like:

• The celestial zodiac is a circle

So context and word grouping help.

A common classroom pronunciation would be roughly:

• ZOH-dee-ah-kus KEER-koo-loos kai-LES-tis est

If using a more classical-style pronunciation, c is always hard, like k, so:

• circulus sounds like keer-koo-lus
• caelestis begins with a hard k sound

Also:

• ae is often pronounced like ai in classical pronunciation
• so caelestis is roughly kai-LES-tis

Pronunciation systems vary somewhat, but the hard c is important.

It follows a very common pattern:

• subject + predicate noun with adjective + est

More specifically:

• Zodiacus = subject
• circulus caelestis = predicate noun phrase
• est = linking verb

So this is a simple equational or identifying sentence:

• X is Y

That pattern is extremely common in Latin and very useful for beginners to recognize.