Rabbi Gettinger had a particular interest in Bein Hashemashos, and spent many years delving into this topic. The crowning achievement of these studies was the publication of his magnum opus “Mnach Yoma,” a ground-breaking discourse on Rabbeinu Tam's opinion on Bein Hashemashos. You may order a copy from Ezras Torah at this link.
His understanding of Bein Hashemashos was premised upon his view of how the Sages understood cosmology, which he described in "Elements of Talmudic Cosmology." This article was published in 1989 in a collection of essays in honor of the 60th birthday of Rabbi Yehuda Cooperman, founder of Michlalah-Jerusalem College. For a more complete presentation of Rabbi Gettinger's views on Bein Hashemashos, please click here.
In Mesechta Shabbos 49b, there is an opinion that the 39 melachos correspond to the number of times that the word melachah (and its variants) appears in the Torah. Rabbeinu Chananel comments that there are 61 instances of the word melacha in the Torah, and then explains which of these are disregarded to arrive at the number stated by the Gemara. In his article "Concerning the 39 Melachos of the Torah" (HaDarom, Tishrei 5726 (1965), pp. 159-80), Rabbi Gettinger points out that the word melacha appears 65 times in the Torah, establishes that the Rabbeinu Chananel originally had the correct figure, and offers a novel explanation -- compelling in its simplicity -- for how the Rabbeinu Chananel came to be corrupted. To hear a series of shiurim by Rabbi Gettinger explaining this article, please click here.