Peel the pears and, with a melon baller or small knife, remove the core and blossom end (bottom) of each pear.
Place the pears, sugar, orange peels, cinnamon stick, ginger and 6 cups water in a 6 or 8-quart Dutch oven. Bring to a boil and reduce the heat to low. Cover and simmer until the pears are tender, about 15 minutes. With a slotted spoon, transfer the pears to a large bowl.
Heat the pear poaching liquid to boiling over high heat and cook, uncovered, over high heat until the syrup is reduced to about 3 cups, about 15 minutes.
Gently pour the hot syrup through a strainer over the pears. Transfer the cinnamon stick, orange peel strips and 3 slices ginger to the syrup around the pears. Cover and refrigerate until well chilled, at least 4 hours, turning the pears occasionally. Serve the pears with the syrup.