You need a semicolon between the words concept
, 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
is a conjunction.
There are three basic kinds of sentences:
- Simple - However, this was when I realized that astronomy was simple in its concept. - That's one thought from start to finish.
- 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).
- 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.