The Smithsonian Astrophysical Observatory Solar Constant Program

TLDR: In this article, the same data set has been used by other authors to show that the sun is constant in output and that the observed variations are explained by errors in the data reduction scheme which failed to remove all the effects of atmospheric extinction or by improper changes in the radiation scale.

Abstract: The Astrophysical Observatory of the Smithsonian Institution (APO) made measurements of the solar constant at many locations on the earth's surface from 1902 to 1962. These measurements have been interpreted by various authors to show that the sun has both a long-term secular change in brightness and cyclic variations. The same data set has been used by other authors to show that the sun is constant in output. The APO solar constant data are reexamined in this review in order to clarify what the APO staff did and what their results say about the behavior of the sun. There are serious problems with the internal consistency of the APO solar constant measurements indicated by the general lack of a common signal between stations or between different measurement methods. If the overall data set is considered, there is no evidence for cyclic variations or any long-term trend in the solar constant greater than a few tenths of a percent. Overall the solar constant appears to be constant to within about 0.1% over the period 1923–1954. Most of the observed variations are explained by errors in the data reduction scheme which failed to remove all the effects of atmospheric extinction or by improper changes in the radiation scale. The solar constant values are independent of solar activity. Although this null conclusion based on the examination of this data set is not encouraging as an easy explanation for climatic change during the twentieth century, the conclusion is valuable in that it sets constraints on the required long-term accuracy and reproducibility needed in the upcoming satellite observations of the solar constant.