The phrase "day sun" refers to the sun that shines during the day. It's another way of saying the sun we see when it is daylight.
Sentences with «day sun»
Heavy - duty all - day sun protection calls for the big guns. (mindbodygreen.com)
It isn't strong enough to use alone for all - day sun exposure but is a great every day option for mild sun protection and skin health. (wellnessmama.com)
For example: you write For a cool autumn day the sun is likely providing 100 W / m2 of energy. (theinconvenientskeptic.com)