1Cut the orange in half and squeeze the juice into a large pot. Cut the rest into chunks and add it to the pot along with the rhubarb and water; bring to a boil. Reduce heat, cover and simmer for about 15 minutes, until the rhubarb is very tender. Mash it a bit with a potato masher.
2If you have a jelly bag, pour the mixture into it and suspend it over a pot. Otherwise, strain it through a fine mesh seive (it will still be cloudy - who cares?) or do as I did - pour it into a cheesecloth, gather it up and rig it up to the cabinet with a rope in order to suspend it over the pot. Let it stay that way for a few hours or overnight.
3In a large pot, add the pectin to the juice (or not), add the sugar and bring to a full rolling boil. Boil hard for a full minute. If you're not using packaged pectin, you could test it - put a bit on a cold plate or somehow cool it down and see if your finger leaves a trail in it - or if you have a candy thermometer, cook it to 210 degrees. Turn off the heat and skim any foam off the surface.
4Ladle into hot jars (I just wash them or run them through the dishwasher - after all, when boiling jam is ladled into them they heat up pretty quick!) and seal.