My first thought on seeing the photo of the painting without its current title was that it was a Sunset with Venus through a smoggy London atmosphere.
Looking at the Tate gallery's catalog entry for it indicate that it is either Turners first or second attempt at painting and that there is indeed some suggestion that it might be a sunset.
But as others have said its a painting, and the notes indicate it was not painted from life but from a sketch.
The Tate is a great site to visit if you like this period in art.
