In general, the Chachamim give these specific examples because, in lieu of sophisticated equipment capable of giving an accurate measure of the moment of sunset and nightfall, these were periods that people were familiar with.

1. "Ani":

a. Rashi (2b, DH mishe'he'Ani) writes that the pauper does not possess a candle to light up his meal. The Me'iri writes that this implies that this Zman is before the stars come out.

b. According to Rashi, it appears that "Ani" is an early time, while according to the Rashba it seems to be later. The Rashba writes that since the pauper hires himself out to others as a worker, he does not come home until at least after sunset.

c. The Me'iri himself makes the time of "Ani" even later and points out that there is something that can be learned from the statement that the "Ani" is eating his bread with salt. This teaches that this time is very late; it is after the stars emerge. This is because the pauper stops working when the stars emerge and only then comes home. However, once he arrives home he can start eating immediately because all he has to eat is bread and salt which require no preparation and are readily bought. The Me'iri writes that the time when the pauper eats his bread and salt is identical to the time when people come in to eat their bread on Erev Shabbos when the meal has already been prepared earlier. This time is similar to the time of the pauper who does not need time to prepare his meal even on a weekday. However, since in those times the synagogues were far from their houses and it took time to walk home, the time that people eat on Erev Shabbos is the same time that the pauper starts his meal.

Kol Tuv,

Dovid Bloom