  The Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric
Observatory observed the transits of Mercury on 2003 May 7 and 2006 November 8.
Contact times between Mercury and the solar limb have been used since the 17th
century to derive the Sun's size but this is the first time that high-quality
imagery from space, above the Earth's atmosphere, has been available. Unlike
other measurements this technique is largely independent of optical distortion.
The true solar radius is still a matter of debate in the literature as measured
differences of several tenths of an arcsecond (i.e., about 500 km) are
apparent. This is due mainly to systematic errors from different instruments
and observers since the claimed uncertainties for a single instrument are
typically an order of magnitude smaller. From the MDI transit data we find the
solar radius to be 960".12 +/- 0".09 (696,342 +/- 65 km). This value is
consistent between the transits and consistent between different MDI focus
settings after accounting for systematic effects.
