It seems the Mods have had a clear out of the Q & A for this sentence, but to summarise: This is an odd use of "ver" over "mirar".
"Ver" normally translates as "to see" whereas "mirar" normally translates as "to look", but the key thing to remember is that "ver" describes your eyes receiving light, whereas "mirar" describes the act of directing your eyes towards light.
As such, both "ver" and "mirar" can translate into English as "to watch" depending on which perspective is taken. Eg:
Are you watching TV? ¿Estás viendo la tele? (are you seeing the TV's images)
Are you watching TV ¿Estás mirando la tele? (are you looking at the TV)
Generally Spanish uses "ver" in this example as the act of receiving images is paramount, but "mirar" could also be used.
In the above DL sentence, however, the use of "looking at" explicitly describes the act of directing your eyes at the sun. As such "¿Estás mirando el sol?" would fit best with the English.
Alternatively, the English could be altered to "Are you seeing the sun?" but even within context this would be interchangeable with the far less awkward "Do you see the sun" or "¿Ves el sol?" The other English alternative for "¿Estás viendo el sol?" would be "Are you watching the sun?"
Firstly, this DL sentence is an odd employment of "ver", but I'll address that separately. More specifically regarding your question, neither "ver" nor "mirar" require the directional preposition "a" in order to say "look at".
There will be cases where you see them followed by an "a", but they will either be when the specific direction of the looking is stated:
Mira a la izquierda - He looks to the left.
Or when the personal "a" is required before a human or humanised direct object:
Ve a la mujer - He sees the woman but Ve la silla - He sees the chair.
Yep, that was an unnecessarily confusing example. I've altered it now. For the record:
La papa - The potato.
El Papa - The Pope.
El papá - The dad.
Are you looking at the sun? Nope, and why would I? I'll become blind and never finish this lesson.
checked my computer. it's fine. the audio, though, on this program? works sometimes, sometimes no. like now: not!