Measuring the Solar Diameter from Space

Image of the lunar transit occulting the solar disk (left) and GOES-EXIS data during 2018 transit (right). The dip in signal strength indicates the darkening due to transit.

Does the size of the Sun change over time? The answer to this question provides insight into the source of the solar cycle (cf. Sophia & Li, 2001).

A recent study by Thuillier et al. (2017) used the bright object sensor on the PICARD mission to measure the solar diameter during lunar transits. The celestial geometry of the Sun, the Moon, and the spacecraft can be calculated very precisely, and an accurate measurement of the diameter of the Sun can be determined from any such eclipse.

This project will use measurements from the Solar Radiation and Climate Experiment (SORCE) and the Geostationary Operational Environmental Satellites (GOES-16 and GOES-17). SORCE measured eclipses for more than a full solar cycle. The GOES instruments can expand this measurement to the extreme ultraviolet, and thus measure the changes of the outer atmosphere. GOES measurements started in 2017, but will continue for many years into the future.

Further Reading:
Sophia, S. & Li, (2001) JGR, 106, 12969
Thuillier, G. et al. (2017) A&A, 603, A28