Okay, I'm not sure if they are "rules" or "grammatical", but here they go:
After a while our necks began to ache, so we sat on the rest chairs behind (us). I liked it here. The air was fresh and there was a low hip roof to shelter us from the wind.
To give myself something to do, I searched for constellations. I wasn't an expert (on it); my knowledge was limited to horoscope columns and astronomy documentaries.
As an author, would you omit the bolded parts? Why or why not?