We study the temporal autocorrelation function (ACF) of global solar oscillations. It is well known that the “large frequency separation” is proportional to the solar acoustic radius. We analyze the ACF of MDI and GONG spherical-harmonic-coefficient time series for degrees ℓ=0−3. Acoustic radius measurements obtained from the first dominant peak locations of the ACF show a significant anticorrelation with solar cycle. This technique can be a useful tool to search for stellar activity.