On the constancy of the solar diameter

Why does the solar luminosity vary and could it change on human timescales by enough to affect terrestrial climate? As important: as these questions are, we lack answers because we do not understand the physical mechanisms responsible for the solar irradiance cycle. Progress here depends on discovering how changes in the solar interior affect energy flow from the radiative and convection zones out through the photosphere. Measurements of small changes in the solar radius are a critical probe of the Sun's interior stratification; they can tell us how and where the solar luminosity is gated or stored. Here we report results from a sensitive 3 yr satellite experiment designed to detect solar diameter fluctuations.