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The Eddington approximation is a simplification to stellar structure models that captures the fact that the diffusion of energy via the random walks of photons passed from atom to atom, inherent in the radiative transfer within stars, results in a bit more outward energy-transfer than inward. It is due to this imbalance that the photosphere receives the energy to make the star shine. The method merely explicitly presumes that from any point, slightly more energy is sent in any of the outward directions ("upward" within the star) than inward directions. It is one of the approximations developed to make the modeling of stars practical and can be considered an example of a two-stream approximation. The method has been refined over time, and variants have been developed that trade away some of the simplicity for more realism.