Astrologia zodiacum et sidera explicat.

The subject is astrologia.

It is in the nominative singular, which is the case normally used for the doer of the action. So astrologia is the thing doing the explaining.

The direct objects are zodiacum and sidera.

They are the things being explained:

• zodiacum = the zodiac
• sidera = the stars / constellations / heavenly bodies

Both are in the accusative case, which is the case commonly used for direct objects.

Because only astrologia is the subject.

The two other nouns, zodiacum et sidera, are not subjects. They are the objects of the verb. So the verb agrees with astrologia, not with them.

• astrologia = singular subject
• explicat = singular verb, he/she/it explains

Explicat is:

• present tense
• active voice
• indicative mood
• third person singular

It comes from explicare, meaning to explain, unfold, or set forth.

So explicat means explains.

Because Latin uses case endings to show the role of each word in the sentence.

• astrologia is the subject, so it is nominative
• zodiacum and sidera are direct objects, so they are accusative

This is one of the main differences from English. English mostly depends on word order; Latin depends much more on case endings.

Yes. Latin word order is fairly flexible because the case endings already show what each word is doing.

For example, these would mean essentially the same thing:

• Astrologia zodiacum et sidera explicat.
• Zodiacum et sidera astrologia explicat.
• Explicat astrologia zodiacum et sidera.

The usual effect of changing the order is not to change the basic meaning, but to change emphasis or style.

Because Latin does not have definite or indefinite articles like English the, a, or an.

So:

• astrologia can mean astrology
• zodiacum can mean the zodiac
• sidera can mean the stars

You understand whether English needs the or not from the context.

Because they refer to different kinds of things grammatically:

• zodiacum is treated as one whole thing: the zodiac
• sidera is plural: stars, constellations, or heavenly bodies

So the Latin is naturally saying that astrology explains the zodiac as a single system and the stars as multiple objects.

Sidera is a neuter plural form of the noun sidus, sideris.

Important points:

• dictionary form: sidus
• gender: neuter
• sidera = nominative or accusative plural

In this sentence it must be accusative plural, because it is one of the direct objects of explicat.

Yes, grammatically it is feminine.

That does not mean astrology is biologically female. In Latin, every noun has a grammatical gender, and astrologia belongs to the first declension, which is very often feminine.

Its gender matters when adjectives agree with it, but here there is no adjective modifying it.

Et means and.

It joins the two direct objects:

• zodiacum
• sidera

So zodiacum et sidera means the zodiac and the stars.

Yes. Sidus can refer not only to a star in the narrow modern sense, but more broadly to a heavenly body, constellation, or starry sign, depending on context.

So in a sentence about astrology, sidera may suggest something a little broader than just physical stars.

From the endings.

• astrologia has a nominative singular ending
• zodiacum has an accusative singular ending
• sidera has an accusative plural form here

That tells you that astrologia is the subject, while zodiacum and sidera are objects.

So even if the order changed, the endings would still guide you.

A classroom-style pronunciation would be roughly:

as-tro-LO-gi-a zo-di-A-cum et SI-de-ra EX-pli-cat

A more classical-style pronunciation would sound approximately like:

• astrologia = as-tro-LO-gi-a
• zodiacum = zo-di-A-kum
• et = et
• sidera = SI-de-ra
• explicat = EKS-pli-kat

The stress normally falls like this:

• astrologia? Actually no: astrologia
• zodiacum
• sidera
• explicat

So one natural reading is:

Astrológia zodiácum et sídera éxplicat.