Learn Hardcore Finnish: Yötaivas on kirkas, ja galaksi näkyy melkein valkoisena nauhana ilman teleskooppia. - The night sky is clear, and the galaxy is visible almost as a white ribbon without a telescope.

In practice, yötaivas is the normal, idiomatic way to say night sky.
Writing yö taivas as two words would look wrong to native speakers in this context.

Could you also say yön taivas instead of yötaivas? Is there a difference?

You can say yön taivas, but it sounds a bit more literal or poetic and is less common for the general concept.

yötaivas = the standard, compound noun for the night sky (a normal dictionary word)
yön taivas = literally “the sky of the night” (genitive + head noun)

yön taivas might appear in poetry or in stylistic language, but in everyday usage yötaivas is the natural choice for night sky.

What exactly does näkyy mean, and how is it different from nähdä or näkee?

näkyä (3rd person: näkyy) = to be visible, to be seen (intransitive)
nähdä (3rd person: näkee) = to see (transitive, someone does the seeing)

In the sentence:

galaksi näkyy = the galaxy is visible / the galaxy can be seen

It doesn’t say who is seeing it. It just states that it’s visible.

If you used nähdä / näkee, you’d need a subject who is doing the seeing:

Ihminen näkee galaksin ilman teleskooppia.
A person sees the galaxy without a telescope.

Here, the subject ihminen does the action. With näkyy, the galaxy itself is the thing that appears/is visible.

Why is there a comma before ja in “Yötaivas on kirkas, ja galaksi näkyy…”?

Finnish uses commas between independent clauses even when they’re joined by ja (and).

Here we have two separate clauses, each with its own verb:

Yötaivas on kirkas – The night sky is bright.
galaksi näkyy melkein valkoisena nauhana ilman teleskooppia – the galaxy is visible…

Because both parts could be full sentences on their own, Finnish punctuation normally puts a comma before ja:

Yötaivas on kirkas, ja galaksi näkyy…

What case is valkoisena nauhana, and what does the ending -na/-nä mean?

Both valkoisena and nauhana are in the essive case.

valkoinen → valkoisena
nauha → nauhana

The essive -na / -nä often expresses:

state/form/role: as, in the form of, in the state of

So melkein valkoisena nauhana means roughly:

as an almost white band
in the form of an almost white band

It describes how the galaxy appears in the sky.

Why are both valkoisena and nauhana in the essive? Could only one take the -na ending?

In Finnish, adjectives normally agree in case with the noun they modify.

noun: nauha → nauhana (essive)
adjective: valkoinen → valkoisena (also essive)

Since valkoinen describes nauha, they both go into the same case:

valkoisena nauhana (correct)
valkoinen nauhana (wrong)
valkoisena nauha (also wrong)

So both must use -na/-nä to match grammatically.

Why is it ilman teleskooppia with teleskooppia in the partitive case?

The preposition ilman (without) always takes the partitive case:

ilman maitoa – without milk
ilman rahaa – without money
ilman teleskooppia – without a telescope

So:

teleskooppi (basic form)
teleskooppia (partitive singular after ilman)

This is just a fixed rule: ilman + partitive.

Why doesn’t Finnish use words like “a” or “the” before galaksi and teleskooppia?

Finnish has no articles like English a/an or the.

Definiteness/indefiniteness (a vs the) is expressed by:

context
word order
sometimes by using demonstratives like se (that/it) or tämä (this)

In the sentence:

galaksi näkyy… can mean the galaxy is visible… or a galaxy is visible…, depending on what is already known in the context.
ilman teleskooppia = without a telescope (because that’s the natural interpretation)

The bare noun is enough; Finnish doesn’t need extra words for a/the.

What does melkein do here, and where can it go in the phrase melkein valkoisena nauhana?

melkein means almost / nearly. Here it modifies valkoisena:

melkein valkoisena = almost white (in colour)

Natural placement is:

…galaksi näkyy melkein valkoisena nauhana…

Other spots are possible but change nuance or sound odd:

galaksi melkein näkyy valkoisena nauhana – the galaxy almost appears as a white band (suggests it’s barely visible at all)
galaksi näkyy valkoisena melkein nauhana – sounds wrong in Finnish

So to express “almost white band”, keep melkein right before valkoisena.

Could you say “galaksi on melkein valkoinen nauha” instead of using näkyy melkein valkoisena nauhana?

You can grammatically say:

Galaksi on melkein valkoinen nauha.
The galaxy is an almost white band.

But the meaning shifts:

galaksi näkyy melkein valkoisena nauhana
→ how it appears / looks in the sky
galaksi on melkein valkoinen nauha
→ makes it sound like the galaxy really is (in essence) an almost white band

The original sentence focuses on appearance to an observer, so näkyy … nauhana is more natural.

Why is it Yötaivas on kirkas and not something like Yötaivas on kirkas taivas?

In Finnish, to say “X is [adjective]”, you normally use:

X on [adjective]

So:

Yötaivas on kirkas. – The night sky is bright/clear.

You usually don’t repeat the noun:

Yötaivas on kirkas taivas.
would sound redundant, like “The night sky is a bright sky.” (possible, but stylistically odd here)

A simple adjective as a predicative is the normal pattern.

Could the word order be “Galaksi näkyy melkein valkoisena nauhana ilman teleskooppia” at the beginning of the sentence?

Yes. Word order is quite flexible in Finnish, especially for emphasis.

Both are fine:

Yötaivas on kirkas, ja galaksi näkyy melkein valkoisena nauhana ilman teleskooppia.
– neutral: first say the sky is bright, then mention the galaxy.
Yötaivas on kirkas, ja melkein valkoisena nauhana ilman teleskooppia näkyy galaksi.
– more marked; emphasizes the way it appears, then reveals it’s the galaxy.

The original order (subject galaksi first in its clause) is the most neutral for learners.

Why isn’t there a pronoun like se (it) before näkyy, as in “ja se näkyy…”?

Finnish often drops subject pronouns when the subject is clear from context or already given in the sentence.

Here, the subject galaksi is explicitly present:

…ja galaksi näkyy…

So adding se is unnecessary:

ja se näkyy would usually refer back to something already mentioned; here galaksi is already stated right after ja, so se isn’t needed.

If you had se without repeating galaksi, then it would mean “and it is visible…” referring back to some earlier noun.

Is galaksi the usual word for something like the Milky Way, or would Finns say Linnunrata instead?

galaksi = a galaxy in general (any galaxy)
Linnunrata = the proper name of our galaxy, the Milky Way

In this kind of descriptive sentence about what you see in the night sky, many Finns might specifically say:

Linnunrata näkyy melkein valkoisena nauhana ilman teleskooppia.

Using galaksi is still understandable and correct, but it sounds more general, as if talking about “a galaxy” rather than our specific Milky Way.