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Baily's Beads Observations during the Total Solar Eclipse 2017 August 21
JOA 2018, Vol. 8, No. 3, Page 19 Author(s): Konrad Guhl, Andreas TegtmeierAbstract: Measuring the angular solar diameter and calculating the real diameter, taking into account the Earth-Sun distance, has been a fundamental challenge for astronomers for more than two thousand years. After micrometer, heliometer or transit measurements, astronomers found one of the best ground-based methods for finding the solar diameter: The observation of the disappearance and reappearance of the remaining sunlight in the valleys on the lunarlimb during total or annular solar eclipses. Due to the fact that Francis Baily (1774-1844) was one of the first who described the tiny points of light on the lunar edge during a total eclipse,the technique was named Baily's Beads observation. Such observations have been a focus ofactivity of IOTA and IOTA/ES for many years. The aim was a measurement of the solar diameter and detection of possible variations. Following the agreement at the ESOP XXXI (held 2012in Pescara, Italy) the measurement program will end in 2017 due to the better precision of spacecraft observations since 2012. This ensures some overlap in the data sequences from bead observation and spacecraftborne observations, for 5 years after the start of the PICARD-spacecraft mission. In 2017, IOTA/ES organized an expedition to the edges of a total solar eclipse (TSE) on August 21, where bead observation ispossible. Additionally, some US observers (IOTA) observed this event; a separate report is expected.