Usamos el planisferio bajo el cielo nocturno.

It can be either, depending on context.

• Present: we use
• Preterite / simple past: we used

In Spanish, usamos has the same form for:

• nosotros/nosotras present of usar
• nosotros/nosotras preterite of usar

So by itself, the sentence is ambiguous. Context tells you which one is meant.

Examples:

• Cada noche usamos el planisferio. = present, habitual
• Anoche usamos el planisferio. = past, one completed event

Because Spanish often drops subject pronouns when they are clear from the verb form.

• Usamos already tells you the subject is we (nosotros/nosotras).

So Spanish usually says:

• Usamos el planisferio...

rather than:

• Nosotros usamos el planisferio...

You can still add nosotros for emphasis or contrast:

• Nosotros usamos el planisferio, pero ellos usan una app.

Spanish uses the definite article more often than English.

Here, el means the, and it appears before both nouns:

• el planisferio
• el cielo nocturno

This is normal in Spanish when referring to a specific thing or a thing understood from context.

A learner might expect something closer to English style, but Spanish usually prefers the article:

• Usamos el planisferio = We use the planisphere
• bajo el cielo nocturno = under the night sky

In this sentence, planisferio most likely means a planisphere, especially an astronomical star chart used to identify stars and constellations.

That makes sense with:

• bajo el cielo nocturno = under the night sky

Be careful: planisferio can also mean a world map in other contexts. But in this sentence, the astronomy meaning is much more natural.

Bajo means under or beneath.

In this sentence:

• bajo el cielo nocturno = under the night sky

Yes, you could also say debajo de, but the meaning and style are a bit different.

• bajo el cielo nocturno sounds smooth and natural, even slightly literary
• debajo del cielo nocturno is understandable, but less idiomatic in this kind of sentence

A useful rule:

• bajo + noun phrase is common for general location or poetic description
• debajo de often feels more physical or literal, as in being underneath an object

Examples:

• bajo la mesa = under the table
• debajo de la mesa = underneath the table

In Spanish, descriptive adjectives usually come after the noun.

So:

• el cielo nocturno = the night sky

That is the normal word order.

You can sometimes move adjectives before the noun, but that usually changes the tone or emphasis and often sounds more literary or poetic. For a learner, the safest pattern is:

• noun + adjective
• cielo nocturno
• noche tranquila
• mapa estelar

Yes. Spanish word order is more flexible than English, although the original version is very natural.

Original:

• Usamos el planisferio bajo el cielo nocturno.

You could also say:

• Bajo el cielo nocturno, usamos el planisferio.

This puts more emphasis on the setting: under the night sky.

Both are correct. The original order is more neutral; the fronted version is more stylistic or emphatic.

Yes, that is the main reason.

• el planisferio
• el cielo

Many Spanish nouns ending in -o are masculine, though there are exceptions in the language as a whole.

Here both nouns are masculine, so the article is el.

Also, the adjective agrees with the noun:

• el cielo nocturno
• If the noun were feminine, the adjective would usually change too:
  • la noche nocturna would be grammatically possible, though not a natural phrase

It is pronounced with the stress on fe:

• pla-nis-FE-rio

A rough English-friendly guide:

• plah-nees-FEH-ryoh

A few pronunciation notes:

• r in -rio is a soft Spanish r
• the e in fe is a clear vowel, like eh
• Spanish vowels are usually pure and short, not gliding like many English vowels

Because they do not mean exactly the same thing.

• de noche = at night
• bajo el cielo nocturno = under the night sky

So de noche simply tells you when something happens. But bajo el cielo nocturno creates a more vivid image and tells you something about the setting.

Compare:

• Usamos el planisferio de noche. = We use the planisphere at night.
• Usamos el planisferio bajo el cielo nocturno. = more visual, more atmospheric

Yes, it is grammatical and understandable in Spain.

A native speaker might find it a little elevated or descriptive, because bajo el cielo nocturno sounds more literary than everyday speech. But it is still perfectly natural, especially in a context about astronomy, stargazing, or educational writing.

A more everyday version might be:

• Usamos el planisferio por la noche.
• Usamos el planisferio para observar las estrellas.

But the original sentence is absolutely fine, especially if the goal is a vivid image.