Solar radius
- Three values are in circulation. The canonical 959.63 arcseconds at 1 au of Auwers 1891, the IAU 2015 nominal radius of exactly 695,700 km, which is 959.22 arcseconds, and an eclipse radius near 959.95 arcseconds measured from the edge of totality. They differ because they measure different layers of the Sun 1 2.
- Quaglia's flash-spectrum fit is 0.32 arcseconds above the canonical value. That is about 232 km at 1 au. The result depends on the adopted limb and detection model 2 3.
- That 0.32 arcseconds moves each path limit inward by 0.6 to 0.9 km on the ground and costs about 1.6 s of central duration in Quaglia's 2017 fitted-radius example, which is roughly half a second per 0.1 arcseconds 2 4.
- Near a path limit the sensitivity is about ten times the central-line figure, and one second of duration corresponds to about 0.03 arcseconds of radius 2 5.
- The recommended presets retain their provenance. Provide 959.63 arcseconds for conventional-product reproduction and the Quaglia 959.95 ± 0.05 arcsecond fit for its eclipse photospheric model. Publish sensitivity bands with their assumptions 2 5.
What this topic covers
The Sun has no hard edge, so every eclipse computation must choose a number for its radius. This topic traces each value in circulation to the measurement behind it, says which layer of the Sun that measurement records, gives the uncertainty each author quotes, and records which value each predictor uses. It then turns the disagreement into numbers a developer can act on: kilometres of path-limit shift, seconds of central duration, seconds at each contact, and the width of the uncertainty band that should be published with a limit line.
Notes in this topic
- Solar radius values and their provenance: every value, its edge definition, wavelength and uncertainty, and what each predictor uses.
- Effect of the radius on eclipse products: path width, limits, durations, contact times, grazing zones, and how to expose the radius as a parameter with an uncertainty band.
What this topic changes for the pipeline
The pipeline gains one explicit parameter, the solar radius in arcseconds at 1 au, with a default, an uncertainty and a provenance string, and a rule that the umbral products use it while the penumbral products may use the same value without measurable loss. Path-limit and duration outputs gain an uncertainty band derived from ± 0.05 arcseconds. Nothing else in the Besselian or raster formulation changes, since the radius enters only through the cone half-angles and the limb test.
References
- 1preprint Rozelot, J.-P. and Kosovichev, A. (2026) Sharper Than Ever: Do Modern Observations Pin Down the Solar Radius to Converge on New Standards? arXiv:2605.23794 Review tabulating canonical, seismic, photospheric, eclipse and nominal radii with km and arcsecond values and a proposed glossary. Full PDF read. Contains a typo giving the canonical value as 965.63.
- 2peer-reviewed Quaglia, L., Irwin, J., Emmanouilidis, K. and Pessi, A. (2021) Estimation of the eclipse solar radius by flash spectrum video analysis. ApJS 256, 36 Full arXiv PDF read. Accepted-paper Figure 2 and conclusions checked 2026-09-30: 959.63-to-960.00 comparison gives 1.8 s central and 19.3 s Vale duration loss; fitted 959.95 +/-0.05 arcseconds gives 1.6 s central loss. Fit depends on limb and detection model. Accepted-paper PDF SHA256 dea52d9a88577023541769f62b3ddd41c601737d64ef7afdd64ab7fe31302e95.
- 3peer-reviewed Lamy, P., Prado, J.-Y., Floyd, O., Rocher, P., Faury, G. and Koutchmy, S. (2015) A novel technique for measuring the solar radius from eclipse light curves: results for 2010, 2012, 2013, and 2015. Solar Physics 290, 2617 Seventeen photometer light curves at 540 nm with Kaguya limb profiles: 959.99 ± 0.06 arcseconds (696,246 ± 45 km). Abstract read on the Springer page; full text paywalled.
- 4trade Dunham, D. (2024) April 8th total solar eclipse, the ultimate lunar occultation, updated 2024 May 6 (IOTA) IOTA's 2024 campaign page: Jubier's map uses the 1976 radius, Irwin's 959.95 ± 0.05 limit, the 0.7 km and 10 s corrections, the 2 km umbral-depth rule, and IOTA's retraction of radius variation. Read via curl on 2026-09-15.
- 5peer-reviewed Wright, E. and Young, C. A. (2024) A raster-oriented method for creating eclipse maps. AJ 168, 163 Section 6.4 gives NASA's position: figures assume 696,000 km, nominal 695,700 km is unsuitable, eclipse values 959.99, 959.95, 959.98, 960.01 listed, Irwin's map shifted the northern limit several city blocks, 1 s of duration near a limit equals 0.03 arcseconds. Read from a saved copy of the IOP HTML (var/downloads/iop_ad6b23_wayback.html).