Appendix B: The Effects of Thermal Changes on the "Absolute" Position of the Secondary . ... I will use the same nomenclature for the rod forces here as I used in Appendix A. In Appendix A, the axial force balance equation at the horizon was shown to be . ... The quantity that we really wish to derive is , where is the rod elongation at some initial epoch (i.e. when the rods were first tensioned) and where is the rod elongation at some later time after the rod temperatures had changed. ...