Astrologus sphaeram circumagit et diametrum ostendit.

Because astrologus is the subject of the sentence, and it is in the nominative singular. Its dictionary form is astrologus, a second-declension masculine noun. For many second-declension masculine nouns, the nominative singular ends in -us.

So here astrologus means the astrologer or an astrologer as the one doing the actions.

They are both direct objects, so they are both in the accusative singular, but they belong to different declensions.

• sphaera is a first-declension noun, so its accusative singular is sphaeram
• diametrus is a second-declension noun borrowed from Greek, so its accusative singular is diametrum

So the difference in ending does not mean they have different jobs in the sentence. They have the same job, but they come from different noun patterns.

It often does, which is why this form can look strange to a learner.

Here diametrum is the accusative singular of diametrus. Even though -um is very common for neuter nominative/accusative singular, it is also the normal accusative singular ending for many second-declension nouns.

So in this sentence, diametrum is not a neuter nominative. It is an accusative singular object.

This is a good reminder that you should identify a noun by:

• its dictionary form
• its declension
• its role in the sentence

and not by one ending alone.

Yes, circumagit is one word. It is a compound verb:

• circum- = around
• agit = drives / moves / sets in motion

Together, circumagit comes from circumagere and means something like turns around, spins, or rotates.

Grammatically, it is:

• 3rd person singular
• present tense
• active voice
• indicative mood

So circumagit means he/she/it turns around or is turning around, depending on context.

Ostendit is from ostendere, meaning to show.

In this sentence it is:

• 3rd person singular
• present tense
• active voice
• indicative mood

So here it means he/she shows.

The ending -t is the normal marker of 3rd person singular in the present tense, which is why the verb itself already tells you he/she/it.

Yes. In spelling, ostendit can be either:

• present: he shows
• perfect: he showed / has shown

So how do we know which it is here? By context.

Since the other verb, circumagit, is clearly present, and the sentence sounds like a description of what the astrologer is doing, the natural reading is that ostendit is also present.

So the sentence is understood as describing two actions happening in the same scene:

• he turns the sphere
• and shows the diameter

Latin usually does not need a subject pronoun when the verb ending already shows the person and number.

Both circumagit and ostendit end in -t, which tells you the verb is 3rd person singular: he/she/it.

And since the noun astrologus is already there, the subject is perfectly clear. Latin therefore does not need to add a separate word for he.

If a pronoun were added, it would usually give special emphasis or contrast.

Because Latin, like English, can use one subject for two coordinated verbs.

Here et joins the two actions:

• sphaeram circumagit
• diametrum ostendit

The subject astrologus applies to both verbs, so Latin does not need to repeat it.

This is completely normal. English does the same thing:

• The astrologer turns the sphere and shows the diameter.

We do not normally repeat the astrologer before shows either.

Here et connects two verb phrases or two parts of the predicate:

• sphaeram circumagit
• diametrum ostendit

So et means and, linking the two actions performed by the same subject.

It is worth noticing that et does not only join nouns. It can join:

• nouns
• adjectives
• verbs
• whole clauses

In this sentence, it joins the two actions.

Latin does not have articles like English the and a/an.

So:

• astrologus can mean the astrologer or an astrologer
• sphaeram can mean the sphere or a sphere
• diametrum can mean the diameter or a diameter

Which English article to use depends on the context and on what meaning has already been established.

No. Latin word order is much more flexible than English word order because Latin uses endings to show grammatical roles.

In this sentence:

• astrologus is the subject because it is nominative
• sphaeram and diametrum are objects because they are accusative
• circumagit and ostendit are the verbs

That means Latin can move the words around more freely without changing the basic meaning. This particular order is a fairly natural one, but other arrangements are possible for emphasis.

So the endings, not just the position, tell you who is doing what.