Preheat the oven to 450°. Finely grate 1/2 teaspoon of zest from 1 of the oranges. Using a small, sharp knife, peel all of the oranges and grapefruits, removing all of the bitter white pith. Working over a bowl, cut in between the membranes to release the sections into the bowl. Squeeze the juice from the membranes into a glass measuring cup; you should have 1 cup.