In a large, non-reactive pot, bring oranges, water, sugar, ginger, and lemon juice to a boil. Reduce the heat to medium-high and boil for approximately 25-30 minutes, until the oranges have softened and the volume has reduced by about one-third.
When the marmalade sheets off your spoon or spatula in thick drops, it's done.
Remove the pot from the heat and funnel the finished marmalade into your prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 10 minutes (adjust for altitude if you live above 1,000 feet in elevation).
When the time is up, remove the jars and set them on a folded kitchen towel to cool. When the jars have cooled enough that you can comfortable handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.