Example of the jet PA measurement procedure applied to HST/WFC3-UVIS LS-enhanced images of 3I/ATLAS obtained on 2026 January 14. The jet direction was determined by identifying the initial ridge defined by the brightest pixels emerging from the nucleus and verified by tracing the jet structure over approximately 30 pixels from the photocenter. PAs were measured manually using an angular grid centered on the nucleus. Repeated measurements performed on different image stretches and enhancement parameters yielded consistent results within ±3°, adopted as the conservative uncertainty for all PA determinations. The January 14 image is shown only as an illustrative example of the PA measurement procedure; the PA time series used in the period analysis is reported in Table 1 and covers the HST observations obtained between 2025 November 30 and 2025 December 27.