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 a re, we lack answers because we do not understand the physical mechanisms re sponsible for the solar irradiance cycle. Progress here depends on discover ing how changes in the solar interior affect energy flow from the radiative and convection zones out through the photosphere. Measurements of small ch anges in the solar radius are a critical probe of the Sun's interior strati fication; they can tell us how and where the solar luminosity is gated or s tored. Here we report results from a sensitive 3 yr satellite experiment de signed to detect solar diameter fluctuations.