Emily Dickinson wrote "There is another sky" in conjunction with a letter she sent to her brother, Austin. In the poem, she tries to convince Austin to return from Boston to Amherst. In this context, the poem is about place. Although Amherst is a place in the world, and therefore subject to change, Dickinson compares it to a heavenly place, one where things always remain the same. This is the idea that your home, or home town, might change but it will always be "home." Since that designation of "home" never changes, the place, in a sense, never changes. This is why she uses phrases such as "whose leaf is ever green" and "unfading flowers."
There is a bit of ambiguity. Is Dickinson talking about heaven, Amherst, both, or something else. The last line is "Into my garden come!" This could easily be taken as a sexual (and therefore, incestuous) invitation (to her brother, Austin). However, given that the poem has hymn-like qualities, it seems more likely that this poem is about the nostalgic lure of Amherst ("home") and the everlasting promise of heaven.
This conflation of worldly place and heaven has evidence in the poem. The first line is "There is another sky." The fact that "there" is used indicates an otherworldly place: something not "here." Later, in the poem, the speaker claims "Here is a brighter green." The "here" indicates a worldly place. The combination of "here" and "there" could imply a connection between the world and heaven. This theme is about the sense, or perception, of such places.