Schmidt backbone

The assumption, used in modern sunspot-number recalibration (Chatzistergos et al. 2017), that a historical observer's detection threshold stayed constant across their whole observing career — named for 19th-century observer Julius Schmidt, one of the "backbone" series the method anchors on. First seen in this vault via claim-carringtons-1870-sunspot-drawing-yields-wolf-number-40, where Hayakawa and Teague (2026) cite Carrington's own 1870 data point as a challenge to the assumption.