This short paper contains a discussion on the use of Langley plots for Sun photometer and Brewer spectroradiometer calibration in the presence of a diurnal aerosol cycle. It is shown that with Langley plots alone, it is impossible to correctly identify or remove atmospheric variations having a 24-hour periodicity with an extreme at local noon. Therefore experimentalists must either be able to exclude such a periodicity using additional measurements, or approach different calibration techniques such as the approximation by linear sections.