La contaminación lumínica afecta al cielo nocturno.

Because afectar is commonly used with the preposition a in Spanish: afectar a algo/alguien.

So:

• afecta a el cielo nocturno would be the full form
• a + el = al, so it becomes afecta al cielo nocturno

This is not the same as the personal a you may have learned for people. It is simply the normal pattern of the verb afectar.

Examples:

• La sequía afecta a la agricultura.
• El ruido afecta al sueño.
• La contaminación lumínica afecta al cielo nocturno.

You may occasionally see afectar used without a in some contexts, but for learners, afectar a is the safest and most standard pattern.

Al is a contraction of a + el.

So:

• a = to
• el = the
• al = to the

In this sentence:

• afecta al cielo nocturno = affects the night sky

Spanish always contracts a + el into al.

Compare:

• Voy al parque. = I’m going to the park.
• Afecta al cielo nocturno. = It affects the night sky.

But this only happens with el, not with other articles:

• a la ciudad
• a los animales
• a las estrellas

Spanish often uses the definite article where English does not, especially with general or abstract nouns.

So Spanish says:

• La contaminación lumínica afecta...

even though English usually says:

• Light pollution affects...

This is very normal in Spanish.

Other examples:

• La música me gusta. = I like music.
• La historia es interesante. = History is interesting.
• La contaminación es un problema. = Pollution is a problem.

So the article la does not necessarily mean a specific instance of pollution; it can also refer to the concept in general.

Because lumínica is an adjective describing contaminación, and contaminación is a feminine noun.

In Spanish, adjectives usually agree with the noun they describe in gender and number:

• contaminación → feminine singular
• therefore → lumínica must also be feminine singular

Compare:

• contaminación lumínica
• problema lumínico
• efectos lumínicos

So the ending changes to match the noun.

Because cielo is a masculine singular noun.

That means:

• article: el cielo
• adjective: nocturno

So:

• el cielo nocturno

The adjective nocturno agrees with cielo, just like lumínica agrees with contaminación.

Compare:

• el cielo nocturno
• la actividad nocturna
• los paisajes nocturnos
• las aves nocturnas

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

So:

• cielo nocturno
• contaminación lumínica

This is the usual pattern.

In English, adjectives usually come before the noun:

• night sky
• light pollution

But in Spanish, the standard order is often:

• noun + adjective

Some adjectives can go before the noun for emphasis or with a different nuance, but nocturno normally goes after:

• el cielo nocturno sounds natural
• el nocturno cielo would sound poetic or unusual

Afecta is the third-person singular present tense of afectar.

The subject is la contaminación lumínica, which is singular, so the verb must also be singular:

• La contaminación lumínica afecta...

Conjugation of afectar in the present:

• yo afecto
• tú afectas
• él/ella/usted afecta
• nosotros afectamos
• vosotros afectáis
• ellos/ellas/ustedes afectan

So here afecta means affects.

Yes. Contaminación lumínica is the standard and widely used expression for light pollution in Spanish, including in Spain.

Lumínico / lumínica is a somewhat technical or formal adjective related to light, but in this phrase it is completely normal.

So if you want to talk about the environmental problem caused by artificial light at night, contaminación lumínica is exactly the right term.

You might also hear explanations like:

• contaminación causada por la luz artificial

but that is more descriptive, not the standard set phrase.

Because Spanish often uses the singular to talk about something in a general sense.

Here el cielo nocturno means the night sky as a whole, not one specific piece of sky.

This is similar to English, which also usually says:

• the night sky not
• the night skies

So the singular is the natural choice.

The written accents show which syllable is stressed.

• contaminación → stress on the last syllable: con-ta-mi-na-ción
• lumínica → stress on mí: lu-mí-ni-ca

They are important because they tell you how to pronounce the words correctly and sometimes distinguish one word from another.

Very roughly:

• contaminación sounds like kohn-tah-mee-nah-see-ON
• lumínica sounds like loo-MEE-nee-kah

In Spain, the c before i in contaminación is usually pronounced like the th in thin:

• contaminaTHIÓN

In much of Latin America, it is usually pronounced like s:

• contaminaSIÓN

Both are correct in their own varieties.

Yes, but the original sentence is very natural and standard.

Some possible alternatives are:

• La contaminación lumínica perjudica el cielo nocturno.
• La contaminación lumínica daña el cielo nocturno.
• La contaminación lumínica afecta a la visibilidad del cielo nocturno.

These alternatives change the nuance slightly:

• afecta = affects
• perjudica = harms / is detrimental to
• daña = damages

So afecta al cielo nocturno is a neutral, clear way to express the idea.