Playing with pushouts

[timelets]

Let's play with it a bit.

Astronomers work all their lives. During that time they observe and record positions of all heavenly bodies they see.
Let A be descriptions of all heavenly bodies recorded during one's life.
Let B be all lives.
Let C be all descriptions of all heavenly bodies.

f: A->B maps live slots to all of astronomers' working lives.
g: A->C maps descriptions collected during ones lifetime to all descriptions.

Let D be descriptions of all heavenly bodies ever observed by astronomers. In short, D is all that astronomers know about the universe.