Part 2: So Emerson's sentence begins "But the soul is light." He doesn't mean light in weight, he means light like sunlight or streetlamp light, light that illuminates. What does it illuminate? _I don't know!_ Possibly, "parts of our being." Emerson is saying "Where the soul is, parts of our being experience something like day; where it was, something like night." And I have no idea what that means. This is poetic and philosophical language, almost religious language. I'm ashamed to say that although this essay is very famous, I've never read it and find it unreadable. Two lines from it are very famous: 1) "Trust thyself: every heart vibrates to that iron string." 2) "A foolish consistency is the hobgoblin of little minds..."

"But the soul is light." This is an example of a metaphor; 'soul' and 'light' are compared to each other indirectly. It's saying that 'soul' and 'light' have something in common with each other. "Where it is, is day; where it was, is night" explains the comparison. Where there is (natural) light, it is day. 'It' refers to the soul, meaning that where the soul is, it is day (another metaphor). Likewise, in a place where it is night-time, there was sun-light, but not anymore. This compares the absence of a soul and the night. That is a very complex piece of work to understand. Good luck with it.

Part 1: Let's work up to it in steps, starting with a different idea. The ideas of "is" and "has" can overlap a bit. You are looking at the earth from space. "The blue is water." That means, the places that look blue are places that have water. They are blue because of water. Water is blue. The mineral turquoise is also blue. If we didn't know, we might think the blue of the earth is turquoise. The blue is not turquoise. The blue is water. By definition, the word "ocean" means "the water that covers two-thirds of the earth." By definition, the word "land" means "the part of the earth that is not covered by water." The phrase "where the ocean is" is a noun phrase. It is short for "the places where the ocean is." We can say "where the ocean is, there are fish. Where the land is, there are trees." So, we can also say "where the blue is, there are fish." And we can say "where the blue is, is ocean." This means you find ocean in the places that look blue. Now consider sunlight. It illuminates some parts of the earth, while other parts are dark. Those parts of the earth that have sunlight are experiencing daytime. Those that do not are experiencing nighttime. It is daytime where the sunlight is. We can use "Where the sunlight is" to mean "places that are lit by the sun." We can say "Where the sunlight is, it is daytime." We can also say "Where the sun is, is day." We can say "Where there is no sun, is night." Because of the rotation of the earth, the parts that are dark now were light, so we can say "Where the sun was, is night."