day/night shader

hi i`m very new to UNITY i want to create space simulation there some question that i want to ask 1. ho to created day/night shader do went we see both day and night surface that react to light source

1. i can created orbital planet but i want to make some orbital comet to and as we know the comet in elips orbit

2. can some one tell me how to script camera witch look object after we click that object

sory for my language because i`m a foreign thank any way