Opportunities of solar astrometry with the Spanish solar eclipse of 12 August 2026

ATel #17940; Costantino Sigismondi (ICRANet, Pescara and UPRA, Roma)
on 4 Aug 2026; 15:00 UT
Credential Certification: Costantino Sigismondi (sigismondi@icra.it)

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The Total Solar Eclipse of 12 August 2026 is an occasion to measure the solar diameter to the nearest hundredth of arcsecond, by exploiting the Baily's beads timing. The Baily's beads were discovered in 1842. We need to gather, as Edmond Halley did in the Total Solar Eclipse of 1715 over England, the positions and the timing of totality (nowadays continuous videos) of the observers the forthcoming Eclipse in Spain. Telescopic videos with 50x magnification may contain informations on tens of Baily's beads, while through smartphones without additional optics it is possible to record the two beads dis/appearing on t2 and t3, delimiting the totality's darkness. The accuracy of 0.01 arcsec allows to better understand the solar activity, also on secular basis thanks to historical measures. The most accurate results are obtained by making the camera fixed: steady images video are recommended, and "street tests" can be done by exploiting suspendend street-lamps as occultating disks (ATel#15991). The best locations for Baily's beads are at the Northern or Southern limits of the eclipse shadow's path. The geometrical effects are there enhanced, and the irregularities of the lunar limb interrupt the crescent of the photospheric light, producing a sequence of beads where the Moon grazes the Sun. The higher number of beads is expected at the southern limit of the shadow (e.g. Madrid) because the lunar South pole is rich of craters and high mountains. Less beads will appear at the Northern limit (e. g. Bilbao) because of a smoother lunar North pole. The whole phenomenon encompasses up to three minutes of time. Very limited by geometry is the situation about the beads on the centerline (e. g. Valladolid): they will appear suddendly just before and after the totality for a few seconds, and they require a telescopic vision/recording. Steady videos obtained through smartphones can be also helpful to integrate our data, since they might be indeed better than the manual timings of the past centuries. The Ghost Images (ATel#15991) of the eclipsing Sun may contain some beads, and it is worth to prepare in advance the pointing of the smartphone in order to have the ghost image, instead to avoid it. Summarizing: we need of geolocation of the observers and a continuous video for a trustable relative timing of all the totality and the minutes before and after it. The choice of the observing location and the check if this point is expected to be near the shadow's bands, where the beads phenomena are more enhanced. Astronomical Observtories on the eclipse path are kindly invited to contact the author: the recording strategies may be improved. All videos received will be credited in the science paper after the eclipse.

Interactive eclipse map for 12 August 2026 (X. Jubier)