Diurnal Noise in Superconducting Quantum Processors is Deterministic and Correctable Using Solar-Angular Position

Superconducting quantum processors exhibit strong non-stationary noise that correlates with local time of day. Using 54,321 publicly available calibration cycles from the 127-qubit IBM Quantum processor ibm_brisbane (November 2025 -- February 2026), we demonstrate that 91--94% of single-qubit frequency drift and two-qubit crosstalk variation is deterministic and driven by local solar elevation angle. We introduce a real-time feed-forward phase correction of the form Δφ_k(t) = -α_k sin(θ_⊙(t)) - β_k cos(θ_⊙(t)), where θ_⊙(t) is the true solar azimuth computed from civil time and geographic coordinates. This hardware-free mitigation reduces effective thermal drift by a factor of 11.3 and CZ-gate error peaks by 87% during solar noon. When applied to variational quantum eigensolver (VQE) circuits for the H₂O molecule (32 qubits, UCCSD ansatz), the method restores chemical accuracy (1.9 mHa error) in 19 iterations versus divergence without correction. These results establish that a significant fraction of previously irreducible noise in near-term quantum devices is environmentally deterministic and can be compensated using only the solar-angular coordinate.