Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits

TL;DR

This study uses space-based imagery from SOHO's MDI instrument during Mercury transits in 2003 and 2006 to measure the solar radius with high precision, reducing atmospheric distortion effects.

Contribution

First high-quality space-based measurements of the solar radius during Mercury transits, providing a more accurate and consistent value compared to previous ground-based methods.

Findings

Solar radius measured as 960.12 arcseconds with 0.09 arcsecond uncertainty.

Results consistent across two transits and different instrument settings.

Measurement aligns with some previous estimates, reducing systematic errors.

Abstract

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.