Shabbat 35B

אלא לאכילת תרומה, דלא אכלי כהנים תרומה עד דשלים בין השמשות דרבי יוסי.

Rather, it must be that the reference is with regard to eating teruma. Priests may not eat teruma until twilight is completed, which according to R' Yosei’s opinion is slightly later than it is according to R' Yehuda’s opinion.

אמר רב יהודה אמר שמואל: כוכב אחד ⁠— יום, שנים ⁠— בין השמשות, שלשה ⁠— לילה. תניא נמי הכי: כוכב אחד יום, שנים בין השמשות, שלשה לילה. אמר רבי יוסי, לא כוכבים גדולים הנראין ביום, ולא כוכבים קטנים שאין נראין אלא בלילה ⁠— אלא בינונים.

With regard to the period of twilight, Rav Yehuda said that Shmuel said: When one can see one star in the evening sky, it is still day; two stars, twilight; 3 stars, night. That was also taught in a baraita: When one can see one star in the evening sky, it is still day; two stars, twilight; 3 stars, night. R' Yosei said: This is neither referring to large stars that are visible even during the day, nor to small stars that are visible only late at night. Rather, it is referring to medium-sized stars.

אמר רבי יוסי ברבי זבידא: העושה מלאכה בשני בין השמשות ⁠— חייב חטאת ממה נפשך. אמר ליה רבא לשמעיה: אתון דלא קים לכו בשיעורא דרבנן, אדשימשא אריש דיקלי אתלו שרגא. ביום המעונן מאי? במתא ⁠— חזי תרנגולא. בדברא ⁠— עורבי, אי נמי ⁠— אדאני.

R' Yosei, son of R' Zevida, said: One who performs a prohibited labor during two twilights, one between Friday and Shabbat and one between Shabbat and the conclusion of Shabbat on Saturday night, is liable to bring a sin-offering for performing a prohibited labor on Shabbat whichever way you look at it. Whether we say that twilight is day or night, certainly one of those labors was performed on Shabbat. Rava said to his servant: You, who are not expert in the measures of the rabbis, when the sun is at the top of the palm trees, light the Shabbat lights. His servant asked him: What should we do on a cloudy day, when the sun is not visible at the top of the trees? Rava said to him: In the city, watch the roosters because as evening approaches they sit on their beams. In a field, watch the ravens because they return to their nests as evening approaches. Alternatively, you can watch the plants [adanei] that turn westward in the evening. When they begin to turn westward evening is approaching.