Solar Time, Angles, and Irradiance Calculator: User Manual

the sun is directly overhead, the air mass number is 1, indicating one full atmosphere that the sunlight must pass through. If the sun is low in the sky, the air mass number will be greater than 1 since the angle causes the sun to pass through more atmosphere, resulting in a loss of solar insolation. Finally, the optimal tilt angle (Σ) is ................
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