Revisamos las efemérides antes de la observación astronómica.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• revisamos can mean we review / we check or we reviewed / we checked, depending on context.
• The -amos ending tells you the subject is we.

So Revisamos las efemérides... is perfectly natural without nosotros. You could say Nosotros revisamos..., but that usually adds emphasis, contrast, or clarity.

It can mean either one.

For many -ar verbs, the nosotros form is the same in:

• the present: we review / we check
• the preterite: we reviewed / we checked

So Revisamos las efemérides could mean:

• We review/check the ephemerides
• We reviewed/checked the ephemerides

Context tells you which one is intended. Time words make it clearer:

• Hoy revisamos... = today we review/check...
• Ayer revisamos... = yesterday we reviewed/checked...

Not exactly. In this sentence, revisar usually means:

• to check
• to look over
• to review
• to examine

It is often a false friend for English speakers, especially if you use British English, where to revise often means to study again for an exam. Spanish revisar usually does not mean that.

So here revisamos las efemérides means something like we check/review the ephemerides, not we revise them in the English study sense.

Because efemérides is feminine.

In this sentence, efemérides is a feminine plural noun, so it takes:

• las = feminine plural the

That gives you:

• las efemérides

In astronomy, efemérides refers to ephemerides: tables or data giving the positions of celestial bodies at certain times.

Here it is plural.

• singular: efeméride
• plural: efemérides

That can feel confusing because the word is a bit uncommon, and in astronomy the plural form is especially common.

So in this sentence:

• las efemérides = the ephemerides

Also, outside astronomy, efeméride can mean an important historical date or anniversary.

Because antes de is used before a noun or an infinitive.

Here it is followed by a noun phrase:

• la observación astronómica

So:

• antes de la observación astronómica = before the astronomical observation

A useful pattern is:

• antes de + noun
• antes de + infinitive

Examples:

• antes de la clase = before class
• antes de salir = before leaving

If a full conjugated verb follows, Spanish usually uses antes de que + subjunctive:

• antes de que empecemos = before we start

Because Spanish normally uses an article here when referring to a specific event or activity.

• antes de la observación astronómica = before the astronomical observation

This sounds like a particular observation already known in the context.

If you wanted a more general or indefinite idea, you might say:

• antes de una observación astronómica = before an astronomical observation

So la makes it sound specific.

Because in Spanish, most descriptive adjectives normally come after the noun.

So:

• observación astronómica = astronomical observation

This is the standard word order in Spanish.

Putting the adjective before the noun is sometimes possible, but it usually changes the tone, emphasis, or style. In a neutral scientific phrase like this, observación astronómica is the natural order.

Because the adjective must agree with the noun it describes.

The noun is:

• observación — feminine singular

So the adjective must also be feminine singular:

• astronómica

Agreement works like this:

• libro astronómico = masculine singular
• observación astronómica = feminine singular
• fenómenos astronómicos = masculine plural
• cartas astronómicas = feminine plural

Yes. It is very normal.

Since Spanish often omits the subject pronoun, the sentence often begins directly with the verb:

• Revisamos las efemérides...

This does not sound like a question or an inversion the way it might in English. It is just a normal declarative sentence.

You could also say:

• Nosotros revisamos las efemérides...

But that sounds more emphatic, as if you are stressing we.

In Spain Spanish, it is pronounced roughly like e-fe-MEH-ree-des, with the stress on mé.

The accent mark shows where the stress goes:

• efemérides
• observación
• astronómica

So the stressed syllables are:

• efeMÉrides
• observaCIÓN
• astronÓmica

That written accent is important because it tells you the correct stress pattern.