What stops Nick from calling to Gatsby at the end of chapter one? What does Gatsby’s “trembling” suggest?

The end of Chapter I of Fitzgerald's The Great Gatsby is the first time we actually see Gatsby. He is standing alone on his lawn, looking out over the water. Nick describes him as emerging from the shadow of his house, and describes his movements as "leisurely" (25). His hands are in his pockets and "the secure position of his feet upon the lawn" tell Nick this is Gatsby, the lord of all he surveys (25). Nick is about to call out to him, to introduce himself to his neighbor, but then Gatsby stretches out his arms toward the water, in the direction of a distant green light across the bay. He seems to be trembling. Nick sees this is clearly an intensely private moment and decides to leave Gatsby to it. Then Gatsby, having emerged from a shadow, slips back into one, and Nick is left alone in the night. We do not know why Gatsby might be trembling at this point, but we can feel in that tremble that he is yearning intensely for something, and that something is represented by the green light across the bay.