Bring water, sugar, and star anise to a boil, stirring until sugar is dissolved. Discard star anise and cool syrup. Stir syrup into juice.
Freeze in a shallow metal pan, stirring occasionally (about every 40 minutes) with a fork, until liquid is frozen and granular, about 6 hours. Just before serving, scrape with a fork, breaking up lumps.
•Granita can be made 1 week ahead and kept frozen, covered.