From what I understand, Solstheim is NOT visible from the top of the Throat of the World. This would mean it's over the horizon, of course, and it's a far horizon since you're so high up (I would guess the mountain in question is at least 15k feet, based on the treeline and the fact that Skyrim is in a northern latitude.)

With this in mind: can anyone tell me how long the boat journey is from Windhelm to Solstheim? Using the speed of other boats as an example of a typical cruising speed (boats such as the one that takes you to a particular 'pirate island' and some of the other boats that take you between harbors), this should allow us to make a relatively accurate calculation of the actual distance between Windhelm and Solstheim.