Earth radius from a single sunrise image: a classroom-ready activity

We present a classroom based activity using a sunrise photograph of Mont Blanc shadow taken from Geneva to estimate the Earth radius. By determining the direction of solar rays relative to the local vertical and accounting for atmospheric refraction, students can derive an upper bound approximately 70 percent above the accepted value. The discrepancy provides a concrete illustration of modelling assumptions and observational limitations. The activity combines geometric reasoning, basic trigonometry, and order of magnitude estimation, allowing students to obtain a physically meaningful result from simple observations. Beyond its quantitative aspects, the approach highlights essential elements of the scientific method, including hypothesis formulation, model construction, uncertainty analysis, and comparison with external data, offering a structured introduction to Nature of Science concepts within upper secondary STEM curricula.