Place pears into a large pot over medium heat, and add just enough water to cover the bottom of the pot and keep them from sticking, about 1/2 cup. Cook until the pears are soft, about 30 minutes. Press pears through a sieve or food mill, and measure out 2 quarts of the pulp.
Pour the pear pulp and sugar into a large saucepan and stir to dissolve sugar. Stir in the orange zest, nutmeg and orange juice. Cook over medium heat until the mixture is thick enough to mound in a spoon. When the mixture begins to thicken, stir frequently to prevent scorching on the bottom. This will take about 1 hour.
Ladle the pear butter into hot sterile jars, leaving 1/4 inch of headspace. Remove air bubbles by sliding a metal spatula around where the pear butter touches the glass. Wipe jar rims clean, and seal with lids and rings. Process for 10 minutes in a boiling water bath. The water should cover the jars by 1 inch. Check with your local extension for exact processing times for your area.