The video "A Private Universe" evokes surprise and dismay among educators and scientists by demonstrating that even the brightest students fail to grasp a seemingly simple and fundamental concept--the reason for seasons. This literature review describes the findings of 41 studies that collectively illustrate why the concept proves difficult to learn, what is lacking in the standard sequence of astronomy education, and what promising methods might be brought to bear. Helping students understand the seasons at a deep level can provide a storyline that cuts across different domains of science, touching on global climate zones, the behavior of light, and connecting the model of Earth as a planet in space with observations that students can make on their own. We include a learning progression, intended to guide the development of instructional materials and assessments as well as questions for further research. (Contains 9 figures and 2 notes.)