Most people know something about the sun, that it is the center of a planetary system, that it makes plants grow, that its rays warm and can burn the skin or tan it when exposure is gradual. Physicians know more about the action of the sun's rays on humans, including the direct chronic effects on the skin (premature aging, keratosis, malignant lesions) as well as many indirect effects due to exogenous and endogenous factors.1
Ecologists' interests extend further. Lemon et al,2 noting that there had been a call for a "land ethic" to conserve and prudently use the interface between air and land, recognized the need to understand and quantitatively describe how it works. They developed a simulation model of a simple plant community—a cornfield. Their model treats crops as energy exchange systems and is based on the conservation of energy where the sun is the driving force.