Desde que usamos el planisferio, reconocemos más constelaciones.

Desde que is a very common Spanish way to say since when you are talking about the point in time when something started.

• Desde = from
• que = that/when

Together, desde que introduces the starting point of an ongoing situation:

• Desde que usamos el planisferio... = Since we started using the planisphere...

It often appears with a present tense in the main clause when the result still continues now.

Spanish often uses the present tense to talk about something that started in the past and is still true now, especially with expressions like desde que.

So:

• Desde que usamos el planisferio = since we have been using / since we started using the planisphere
• reconocemos más constelaciones = we recognise more constellations

In natural English, you might prefer:

• Since we started using the planisphere, we recognise more constellations or
• Since we've been using the planisphere, we've been recognising more constellations

Spanish does not always need a perfect tense where English often does.

Yes. Usamos can mean either:

• we use
• we used

The form is the same for the present indicative and the preterite of usar for nosotros.

In this sentence, the context with desde que strongly points to the present meaning:

• since we use / have been using / started using

So here it is understood as part of an ongoing situation, not a completed past action.

Because reconocer means to recognise, not just to know.

• conocer = to know / be familiar with
• saber = to know a fact / know how
• reconocer = to recognise, identify

So:

• Reconocemos más constelaciones = We can identify more constellations
• Conocemos más constelaciones would sound more like we are familiar with more constellations
• Sabemos más constelaciones is not natural here

If the idea is spotting them in the sky and identifying them, reconocemos is the right choice.

Spanish uses the definite article much more often than English.

So Spanish says:

• usamos el planisferio

where English often says:

• we use the planisphere or sometimes simply
• we use a planisphere, depending on context

Here el refers to the planisphere being used in the situation, even if English might not always stress the article in the same way.

Because más constelaciones is being used in a general, indefinite way.

• reconocemos más constelaciones = we recognise more constellations

It does not mean a specific set already known to both speaker and listener. If you said las constelaciones, that would point to specific constellations.

So:

• más constelaciones = more constellations in general
• más de las constelaciones would mean something very different and would not fit here

With desde que, you normally use the indicative when talking about something real or already established, as in this sentence:

• Desde que usamos el planisferio...

That is why usamos is indicative.

In practice, desde que usually refers to a real starting point, so the indicative is the normal choice. It is not one of the common expressions that regularly triggers the subjunctive in everyday use.

Grammatically, Spanish can use the present perfect in some contexts, but Desde que usamos el planisferio sounds more natural here.

Spanish often prefers the simple present after desde que when the action began in the past and still continues.

So the original sentence sounds idiomatic and natural:

• Desde que usamos el planisferio, reconocemos más constelaciones.

Using hemos usado would sound less natural in this kind of general ongoing statement.

Here planisferio refers to a star chart / planisphere, a tool used to identify stars and constellations in the sky.

A learner might wonder because planisferio can also mean a world map in other contexts. But with constelaciones, the astronomy meaning is clearly intended.

So in this sentence, it means the rotating sky chart used for stargazing.

Yes, you could. Identificamos más constelaciones is possible and clear.

The difference is slight:

• reconocemos = we recognise them when we see them
• identificamos = we identify them, perhaps in a slightly more deliberate or technical way

In many contexts, both work. Reconocemos sounds very natural if the idea is becoming better at spotting and recognising constellations in the sky.