So set its Sun in Thee — Slavery

The clue is "I the ships" in line four. Human beings being treated as cargo hints at slaves.

So set its Sun in Thee ¹
What Day be dark to me－
What Distance－far－
So I the Ships may see
That touch－how seldomly－
Thy Shore?

(F.940/J.808)

[1] set its Sun:: to sink the Day's glory; its as the Day in line two. Thee:: the United States of America.
[2] Day be dark:: a dark time (1865) for slaves in America.
[3] Distance far:: slaves were away from home.
[4] I the Ships may see:: "I, the ship's cargo, may see"; or "I may see the ships." The first reading hints at the answer; the second says Dickinson imagined that she saw the slave ships.
[5] touch:: to reach one's heart.
[6] Thy Shore:: support of this land, country.