La fakto, ke satelito orbitas planedon, gravas en astronomio.

Because la fakto means the fact, referring to a specific fact: namely, the fact stated in the following ke-clause.

In Esperanto, la is used when the noun is definite or specifically identified. Here, the clause ke satelito orbitas planedon tells you exactly which fact is meant, so la fakto is natural.

Without la, fakto would sound more like a fact or fact in a more general sense.

Ke means that.

It introduces a subordinate clause:

• La fakto = the fact
• ke satelito orbitas planedon = that a satellite orbits a planet

So the whole structure is:

• La fakto, ke ... , gravas
  = The fact that ... is important

This is a very common Esperanto pattern:

• Mi scias, ke li venos. = I know that he will come.
• Estas klare, ke... = It is clear that...

Esperanto often uses commas to mark subordinate clauses more clearly than English does.

So in:

• La fakto, ke satelito orbitas planedon, gravas...

the ke-clause is set off with commas because it explains what la fakto is.

English sometimes omits such commas, but in Esperanto punctuation with subordinate clauses is often more explicit and regular.

Because the sentence is talking about a satellite in a general sense, not one specific satellite already known to the listener.

So:

• satelito = a satellite
• la satelito = the satellite

The sentence expresses a general scientific fact, so Esperanto leaves it indefinite:

• ke satelito orbitas planedon = that a satellite orbits a planet

If you said la satelito, it would suggest a particular satellite already identified in the context.

The -n ending marks the direct object in Esperanto.

Here:

• satelito is the subject
• orbitas is the verb
• planedon is the thing being orbited, so it is the object

So:

• satelito orbitas planedon = a satellite orbits a planet

This -n ending is one of the most important parts of Esperanto grammar.

Compare:

• La knabo vidas hundon. = The boy sees a dog.
• La hundo vidas knabon. = The dog sees a boy.

The -n helps show who is doing the action and who receives it.

Orbitas is simply the verb to orbit.

So:

• satelito orbitas planedon = a satellite orbits a planet

Esperanto often allows a direct verbal form where English does too. You could express the idea differently, but orbitas is compact and natural.

A more expanded version such as estas en orbito ĉirkaŭ planedo would mean something like is in orbit around a planet, which is similar in meaning but structured differently.

Both are possible in Esperanto, but gravas is a very common and natural way to say is important / matters.

So:

• gravas = is important, matters
• estas grava = literally is important as well

The difference is mostly stylistic:

• Tio gravas. = That matters.
• Tio estas grava. = That is important.

In many cases, gravas sounds a little more idiomatic and concise.

En astronomio means in astronomy.

There is no la because astronomio is being used as the name of a field or discipline, much like in English when we say in biology, in physics, or in astronomy.

So:

• en astronomio = in astronomy
• en la astronomio would usually sound less natural unless you meant some specific astronomy already defined in context.

Yes. Satelito can refer to either a natural satellite or an artificial one, depending on context.

In a sentence about astronomy like this, many readers may first think of a natural satellite, such as a moon. But the word itself is broad.

If you want to be more specific, you can say:

• natura satelito = natural satellite
• artefarita satelito = artificial satellite

The given word order is the most natural:

• La fakto, ke satelito orbitas planedon, gravas en astronomio.

Because Esperanto marks objects with -n, word order is more flexible than in English. However, not all possible orders sound equally natural.

This sentence works best with:

1. the main topic first: La fakto
2. the explanatory clause next: ke satelito orbitas planedon
3. the main statement after that: gravas en astronomio

You could sometimes move parts around for emphasis, but the original version is the clearest and most standard.

Not exactly. Orbitas is simple present tense, so the most direct translation is orbits.

So the clause is:

• ke satelito orbitas planedon = that a satellite orbits a planet

Depending on context, English sometimes uses is orbiting, but Esperanto -as does not automatically mean progressive action. It usually just marks present time.

So in a general scientific statement like this, orbits is the best match.