The article presents simple outdoor experiments for measuring illuminance on inclined surfaces with a luxmeter. A hill model is used to investigate the difference in illuminance between a sunny side and a shady side of a hill, with up to a fivefold difference found between the two sides on a sunny day. The illuminance of the sunny side of the hill at different inclinations is also measured and possible implications for cross-curricular teaching and efficient positioning of solar energy collectors are added. Outdoor optics instruction, cross-curricular lessons, and an inquiry-based approach are suggested to keep students active and motivated.