What verb form is the phrase '見上げたんだ' using?


From the song 'Nirvana' by Tia. I am currently working on translating this
song as an exercise and I noticed that this phrase is different from the verb
forms I already know.

見上げたんだ

The meaning is supposed to be 'I looked up' - but wouldn't you just say
'見上げた'? What does this 'nda' mean? If it's casual speech for です in this case,
why add だ to an already complete sentence?

Thanks very much!




Simply put, it's used for emphasis.

