  In 1893, Wien applied the first two laws of thermodynamics to blackbody
radiation and derived his displacement theorem. Believing that the information
from thermodynamics had been exhausted, Planck turned to statistical ideas in
1900 in order to provide a physical understanding for his
experimental--data-based interpolation giving the Planck spectrum without
zero-point radiation. Here we point out that the third law of thermodynamics
(which was developed in the early years of the 20th century) introduces
additional thermodynamic information regarding thermal radiation. The Planck
spectrum for thermal radiation can be derived from purely thermodynamic ideas
applied to the classical simple harmonic oscillator, since every radiation mode
takes a simple oscillator form. Thermodynamics alone implies the Planck
spectrum including zero-point energy without any need for quantum theory or
statistical ideas. It is pointed out that the inclusion of zero-point energy
involves more natural thermodynamics than its exclusion.
