In a saucepan combine sugar with 1 1/2 cups water; bring just to a boil over medium heat, stirring until sugar dissolves. Remove pan from heat, leave to cool, then refrigerate until cold. Stir in lemon juice and zest. Pour lemon syrup into a square metal cake pan and place in freezer. Freeze 2 hours in all, removing pan from freezer every 30 minutes, stirring and scraping with a metal fork until ice crystals break up. After 2 hours, granita will resemble snow. Scrape into 4 dessert goblets and serve with a spoon and a straw if desired.