La observación astronómica empieza ahora.

Because observación is a feminine noun in Spanish, so it takes the feminine singular article la.

A useful clue is the ending -ción: nouns ending in -ción are almost always feminine.

• la observación
• la información
• la nación

So even though English nouns do not have grammatical gender, in Spanish you have to learn each noun with its article.

Astronómica is the adjective astronomical. It describes observación, and it must agree with that noun in gender and number.

Since observación is:

• feminine
• singular

the adjective must also be:

• feminine
• singular

So:

• observación astronómica = feminine singular
• fenómeno astronómico = masculine singular
• observaciones astronómicas = feminine plural

In Spanish, adjectives often come after the noun, especially when they are descriptive/classifying adjectives.

So:

• observación astronómica literally follows the pattern observation astronomical

This is very normal in Spanish. English usually puts adjectives before the noun, but Spanish very often puts them after.

The written accents show which syllable is stressed.

• observación → stress on the final syllable -ción
• astronómica → stress on nó

These accents are important because Spanish spelling uses accent marks to show pronunciation when the stress does not follow the default pattern.

Very roughly:

• ob-ser-va-CIÓN
• as-tro-NÓ-mi-ca

Empieza is the third-person singular present tense form of the verb empezar.

The subject here is la observación astronómica, which is singular, so you need the él/ella/usted form:

• yo empiezo
• tú empiezas
• él/ella/usted empieza

Also, empezar is a stem-changing verb in the present tense:

• e → ie

So:

• empezar → empieza not empeza

Because empezar is one of many Spanish verbs with a stem change in the present tense.

The change is:

• e → ie

Examples:

• empiezo
• empiezas
• empieza
• empezamos
• empezáis
• empiezan

Notice that nosotros and vosotros usually keep the original stem:

• empezamos
• empezáis

This is a very common pattern in Spanish.

Yes. Empezar and comenzar often mean the same thing: to begin / to start.

So these are both natural:

• La observación astronómica empieza ahora.
• La observación astronómica comienza ahora.

In many contexts, they are interchangeable. Learners usually hear empezar very often in everyday speech, but comenzar is also completely correct.

Because ahora means now, and Spanish often places time expressions like this at the end very naturally.

So:

• La observación astronómica empieza ahora.

is completely normal.

But Spanish is flexible, and you could also say:

• Ahora empieza la observación astronómica.

That version puts more emphasis on now.

Yes. In Spanish, the present tense is often used for something happening right now or at this moment.

So empieza ahora naturally means:

• starts now
• is starting now
• begins now

Spanish often uses the simple present where English might choose either the simple present or the present continuous, depending on context.

In this sentence, yes, la sounds natural and correct because you are referring to a specific event or activity: the astronomical observation.

Without the article:

• Observación astronómica empieza ahora

this sounds incomplete or unnatural in standard Spanish.

Spanish uses articles more often than English in many contexts, so it is important not to drop them automatically.

A simple pronunciation guide is:

la ob-ser-ba-THYON as-tro-NO-mi-ka em-PYE-tha a-O-ra

In standard Spain pronunciation:

• z in observación and empieza sounds like th in thin
• c before i/e also has that th sound in most of Spain

So:

• observación → obserbaTHYON
• empieza → emPYEtha

If you are learning Spain Spanish, that pronunciation is the usual model.