We have already established that the amount of food that is needed in order to establish an eiruv tehumim that would allow a person to walk beyond 2,000 cubits from his home is mazon shtei se’udot – two meals’ worth. In the course of trying to establish an objective definition of that amount, the Mishna (82b) quotes a number of different opinions. Rabbi Shimon rules that it is two-thirds of a loaf of bread, when there are three such loaves to a kav (a measure of capacity, one sixth of a se’ah). As an aside, Rabbi Shimon records other measurement rules that are based on such a loaf, specifically rules of ritual purity – tuma v’tahara.
The Gemara on our daf quotes a baraita that teaches other rules regarding the amount of food that is necessary in other such cases, and explains that these rules do not appear in our Mishna because the quantities are not in precise proportion to one another, which would lead to confusion.
At least part of the confusion stems from the fact that three different systems of measurement were used in the Gemara, all of which are discussed here. These three systems stem from three different historical periods. The “Midbarit” (wilderness) measurement is the one that we are familiar with from its use in the Bible, and it is the basis for establishing all weights and measures in halakha. The “Yerushalmit” (Jerusalem) measurement was used during the Second Temple period, and the “Tzipporit” measurement was established after the destruction of the Second Temple, when the center of Jewish life moved to Tzippori.
It is important to note that the most basic unit of measure – the betza (egg) – did not change. All of the changes mentioned are in other units of measurement. This is because the betza is more or less a natural unit, and because of its objective quality, it is not dependent on communal or societal agreement. Therefore the only units that changed were the ones that were based on general agreement – the log, kav and se’ah.