Just to be nitpicky: In daylight you can't always see to your horizon. If you often go to the grand canyon for example, you can see the other side on most days, but each day your visibility is different. Some days you can't see it at all. This is caused by clouds and smoke, but sometimes the air is just thick with no apparent clouds, smoke, or fog around. The grand canyon is only a mile wide. I thought the grand canyon was a good example because the visibility conditions aren't caused by smog like you find in cities. And even if you can see to the horizon on a clear day, you can't make anything out other than the line where the land hits the sky. In the D&D rules for seeing, I wouldn't consider that good enough. I believe the rules refer to the distance where you can still make things out. You can see a ship come up over the horizon if you look out to sea, but you can't distinguish it from a rock unless you use binoculars or something.