As we have learned, hametz becomes forbidden even before Pesah begins. According to the Gemara, starting at mid-day on erev Pesah, there is a Biblical prohibition against eating hametz. The Mishna (11b) teaches that according to Rabbi Meir one can eat hametz until the end of the fifth hour and should destroy the remaining hametz at the beginning of the sixth hour. Rabbi Yehuda rules that one should finish eating hametz by the end of the fourth hour. The hametz can remain for the duration of the fifth hour but needs to be destroyed at the beginning of the sixth hour.
This diagram shows how the 24 hours of the day were understood by the Talmud, beginning at the top with sunrise, and moving counter-clockwise to mid-day (the 6th hour) and to sunset (the 12th hour). According to this system, the length of a daylight hour changes, with longer hours during the summer months and shorter ones during the winter.
The discussion of the Gemara on our daf revolves around the likelihood that a mistake might be made about the time, which might lead someone to continue eating hametz after its permitted time. It is important to remember that people could not be exact in figuring times, since accurate clocks did not yet exist. Therefore, the possibility of making a mistake – even of several hours – was a distinct possibility. Rava concludes the discussion in the Gemara by saying that the positions of Rabbi Meir and Rabbi Yehuda in the Mishna stem from the concern that erev Pesah will be overcast, and people will not be able to use the sun to judge the time of day. In such a case, even the rudimentary instruments that were used to measure the length of the day – like sun dials – would not be operative.
Rav Pappa explains that even on a cloudy day we still can establish four hours as a permissible time to eat hametz, since that is the normal time for most people to eat. Therefore, even if people are not able to judge time based on observing the sun, they can do so based on their appetites. When they are hungry they know that it is four hours into the day, when it is still a safe time to eat hametz.
As a point of background, during the Talmudic period, the accepted custom was to eat two meals a day – one in the morning and one in the evening. Some people would snack between meals, but on normal days these were the only two times that people ate. (Shabbat was an exception, when three meals were eaten.) The Gemara points out that workers, for example, who leave for work very early in the morning, would postpone their meal until mid-day in order to break up their workday. Most people, however, ate a little earlier at four hours.