In The Return of the King, the chapter "The Black Gate Opens" where Gandalf, the King of Gondor, and their allies make a final war against Sauron before the Morannon we are told:
...the sun now climbing towards the south was veiled in the reeks of Mordor, and through a threatening haze it gleamed remote, a sullen red, as if it were the ending of the day...
But then, moments later (when the sun must be even higher in the sky, farther west, thus farther out of the "reeks of Mordor" and thus stronger), we learn:
...there came striding a great company of hill-trolls out of Gorgoroth.
However, from The Hobbit we see three hill-trolls turned to stone at the first touch of dawn light. Because the sun above the battle before the gates of Mordor was as strong as "the ending of the day," it must also be at least as strong as the last moment of sunlight which we know is enough to literally petrify hill-trolls.
How to explain?