The news to which Emily Dickinson refers in “I'll tell you how the Sun rose” is the dawning of a new day. The word “news” is particularly appropriate here for two reasons. First of all, because a new day has dawned and, secondly, because in Dickinson's day it was quite common for people to read daily newspapers in the morning.

News travels fast, as they say, and it's no different here. In fact, the news of the sun rising travels even faster than most kinds of news for the simple reason that everyone can see the dawn with their own eyes. The sun instantly communicates its arrival for another day, and other features of the natural world, such as the hills, respond accordingly.

The speed at which everyone receives this news is likened to the running of a squirrel. If you've ever seen a squirrel run, especially if it's being chased by a cat, then you'll know just how fast they can be. The comparison is particularly apt given that the squirrel, like the sun, is part of the natural world. So, too, is the humble bobolink, a species of blackbird. The bobolinks respond to the rising of the sun by singing, precipitating the speaker's realization:

That must have been the Sun!