Navel Orange and Lemon Marmalade

• 4 to 4½ pounds navel oranges (about 7), well scrubbed
• 2 lemons, well scrubbed

Prepare for water-bath canning: Sterilize the jars and keep them hot in the canning pot, put a small plate in the freezer, and put the flat lids in a heatproof bowl. (details.)

Use a vegetable peeler to cut the zest from 5 of the oranges, then stack the slices and cut them crosswise into thin julienne strips; you should have about 1½