The daf for 2/27/18 is from Meseches Avodah Zarah 43 and continues the gemara’s discussion on idols and idol worship. The specific issues concern which images are forbidden and incidental use. For example, while the Torah prohibits making “graven images” and to not follow the practices of the other nations, an interesting discussion concerns images of the sun, moon, planets, and stars. It is well known that celestial objects were worshiped as gods (The ancient Egyptian sun god Ra for example), but as previously pointed out in a blog post, the Jewish calendar relies on the use of charts of the moon and stars for computing (or sanctifying) the new moon (for Rosh Chodesh). The gemara recalls (form Meseches Rosh Hashannah), how Rabban Gamaliel used to interview the witnesses who came before him to testify that they has seen a crescent moon, so that the new month could be proclaimed (translation from Sefaria.org The William Davidson digital edition of the Koren Noe Talmud Bavli):
The issue of “mares ayin” (in which the public appearance of an injunction is prohibited, would give Rabban Gamaliel the so-called “benefit of the doubt:, that his charts of the moon and stars were being used to a Torah commandment to proclaim the dates of the festivals.
The Rambam (Rabbi Moses Maimonides) in his Mishneh Torah, Sefer Madda, Hilchos Avodah Zarah, chapter 2:1-2 (translation from Sefaria.org, Mishneh Torah translated by Moses Hyamson, 1937-1949 ; in the public domain) concerning this writes:
The question that concerns the gemara, is whether the use of astronomical charts, necessary for computations for for the cycles of the moon, constitutes avodah zarah. Later in daf 43b, we read (translation is from Sefaria.org The William Davidson digital edition of the Koren Noe Talmud Bavli):
We should point out that a baraita refers to Tannaic teachings that were not incorporated into the codified Mishnah. With regard to the “charts” the Rabban Gamaliel had to examine witnesses testifying about seeing a crescent moon, so that a new month may be proclaimed (see Meseches Rosh Hashannah 24 for a an almost verbatim argument to the ones stated above). The mitzvah to calculate the motion of the planets and stars can be found in a final gemara from Meseches Shabbos 75a (translation from Sefaria.org The William Davidson digital edition of the Koren Noe Talmud Bavli):