We have used the Sydney University Stellar Interferometer to measure the angular diameter of alpha Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, alpha Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967+/-0.066R_{sun}_. We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420+/-170K . This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates, which span over 1000K and were based on either photometric indices or spectroscopic methods. In addition, we have analysed two high-quality spectra of alpha Cir, obtained at different rotational phases and we find evidence for the presence of spots.