lighting and terrain issue

Good day, I am using unity 5.4 and have some problems with terrain and lighting:
I posted this question in the http://answers.unity3d.com section but it seems that this section is not suited for long questions like mine and that’s why I decided to post the question here in the forums instead.

When I create a terrain and add a directional light to act as a sun, only everything in front of the camera is illuminated by the sun. Everything behind the camera is not illuminated. When I move the camera back, the illuminated area grows as you can see in the following video I made:

What is the meaning of this? Can’t a directional light illuminate a 500 x 500 terrain? I am using an ambient lighting value of 1, because i would like to recreate the real life sun effect.

I hope some of you can help me solving my issue as I really like the unity engine so far.

You are using an actual Unity terrain, or just a big mesh?

Make sure the correct shader/material is selected. Make sure Smoothness value is 0.