The best-known hydrodynamic model of a radiating gas orbiting in the gravitational potential of a compact object is the steady-state, thin accretion disk model (Shakura & Sunyaev 1973; Pringle 1981). A central problem for this model is a prescription for the viscosity that is required to drive matter inward and heat it, while transporting angular momentum outward. Initially, the viscosity was modeled using an ad hoc scaling assumption (Shakura & Sunyaev 1973). This model leads to the conclusion that the temperature profile and the conclusion that the inner annulus in the disk dominates the thermal spectrum, because . This result has a striking observational consequence: X-ray astronomy is the window of choice for probing strong gravity near the horizon of an accreting stellar-mass BH.