In this paper I present a heuristic approach to understanding the temperature of aplanet in orbit around a star. To simplify the discussion, I begin by introducing theconcept of the planet’s mean temperature. I proceed to discuss the effect of any atmosphere that might be present and also consider the effect of so- called "greenhousegases" present in this atmosphere. In contrast to the usual discussion of the importance of the effect of the "trapping" of electromagnetic radiation by the greenhousegases, I present the opposing idea that it is the reflection of electromagnetic radiationfrom the sun that is of overwhelming importance. No concepts more advanced thanthose of basic thermodynamics as presented in a high school physics class are used.While the conclusions pertain to any planet in a radiation flux from a star, the obviousapplication is to the so-called global warming experienced by the earth. The conclusions in this brief analysis are not presented as final, but rather open to debate. It ishoped that this analysis will serve as the basis for a detailed understanding of the climate change — if any — that the earth is experiencing. Comments and corrections arewelcomed.