Rav Yechiel Michel Tukatchinsky, mashgiach of Slabodka in Bnai Brak, and founder of YeshivasMekor Chaim in Yerushalayim. In 1925, he published a sefer called Tekufas Hachamoh Uvirchosoh, in preparation for the bracha made when the sun returns to the point at which it began upon Creation. He wrote a sefer called Bein Hashmoshos, published in 1929, which dealt with the International Date Line. In 1941, he changed his mind altogether, as documented in his sefer, Hayomam Bekadur Haaretz.
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His sefer Bein hashemashot was the first sefer to combine deep knowledge of halakha and science wrt to define the end of shabbos precisely. His proposed method of calculation, a depression angle of 8.5 degrees, is used universally. He wrote Ir Hakodesh and the definitive sefer on aveilut, gesher hachaim, and a defense of the heter mechirah. He was married to the granddaughter of R. Isroel Salant. He famously won an argument with R. Yosef chaim Sonnenfeld, when he was a young man 110 years ago and his method of calculating the shittah of the MA’s zemanin is still used today in yerushalayim.