A few pages ago, Stephen looked out at Dublin Bay and thought of the scene at his mother’s deathbed, associating the view with the bile his mother had coughed up into a white bowl. That image is still with him here, “a bowl of bitter waters.”
The cloud covering the sun will appear again in a few chapters, when Leopold Bloom sees the same cloud at the same time from a different part of the city. The observation of the same phenomenon from two different places invokes parallax, an important concept for Ulysses. Parallax is a technique for finding the distance of a remote object, like a planet or star. The wikipedia article will tell you how it works, but the basic principle is that when you see something from two points of view, you can figure out where it really is. Our two eyes automatically use parallax to determine depth in the world around us.
Bloom, who has an active, if uninformed interest in astronomy, thinks about Parallax several times during the day, but it also is a kind of metaphor for Joyce’s method. We see the phenomena of one day in the life of a City from several different perspectives, and we need to take more than one perspective into account to find the real depth of the story.
Rob’s drawing reinforces this idea–we look from a POV that’s different from Stephen’s, and both of us can see the mail boat coming in to the harbor.