Poets often put their own spin on the Ptolemaic Model of the Universe ( geocentric model). Donne is making reference to this here. In his version of the model the Earth is in the center most concentric circle, then the moon, and then heavenly bodies. By juxtaposing "sublunary lovers" (constantly changing lovers) to him and his lover, Donne is saying that his love with More resides in the outermost perfect and unchanging circle.