Grammar help

You should ask your teacher "What should I do there?" That is what they are there for. Not to just tell you "Try again.", but to teach. I don't see anything wrong with the grammar. Maybe use a semicolon instead of a comma. Or end the sentence at "concept" and start the next sentence with "Point". I think it should be "... but there 'were' what seemed..."

PS: I don't see anything wrong with the comma after "concept".

concept:

You need a semicolon between the words concept and point, or a period and then capitalize the word point. You could potentially also use a colon. Personally, I would use a period.

Why? Because these are two separate, independent sentences. They each contain a full thought.

And why would I use a period? Because I think the sentence is running on too long, so at least you would cut it off about one-third of the way in. Shorter sentences (in general) also improve readability.

Oh, and another thing: the part that starts with but is another complete thought. Yet it takes a comma before the word but because but is a conjunction.

There are three basic kinds of sentences:

1. Simple - However, this was when I realized that astronomy was simple in its concept. - That's one thought from start to finish.
   
2. Compound - Point a telescope at a star and look at it, but there was what seemed like hundreds of minor details that included everything... - That's two thoughts, separated by a comma and then a conjunction (conjunctions are words like and, but, or, nor, and for).
   
3. Complex - However, this was when I realized that astronomy was simple in its concept; point a telescope at a star and look at it. - These are two separate thoughts, no conjunction and no comma. The comma + conjunction bit is replaced by just a semicolon.

As for using a colon, that would make sense, too, as the part after the colon would be explaining things. E. g. However, this was when I realized that astronomy was simple in its concept: point a telescope at a star and look at it.

And no matter what, just a comma, with no conjunction, does not separate two full thoughts.

"Was" just sounds wrong to me, because of "hundreds" and "details" being plural. Am I wrong?

"...there were hundreds of minor details..." would be correct. It doesn't seem like "what seemed like" would alter that.

Just to clarify, this is a question of mechanics not grammar which Jespah proficiently answered.

You're right. You know, I didn't even notice that. Good eye!

Haha. Thank you.