Adapted from a shiur by Rav Yitzchak Grossman on Parshas Terumah
(אורך החצר מאה באמה ורחב חמישים בחמישים וקמה חמש אמות שש משזר ואדניהם נחשת (שמות כז:יח
The Gemara (Eruvin 23a) sees this pasuk concerning the measurements of the Mishkan as an asmachta (allusion) to the rabbinic halacha of a karpef: An area 5000 square amos that is lo hukaf l’dirah (not walled for people to live) may not be included as part of an eruv to allow carrying on Shabbos.
It is based on the following points:
- A karpef is similar to a reshus harabim (public domain where carrying is forbidden) in that it is open and not used for housing (Rashi).
- It is derived from the Mishkan like other halachos of Shabbos (Levush).
- Chazal only forbade a karpef larger than the Mishkan’s dimensions (Shulchan Aruch HaRav).
The Gemara states that gardens with seeds (zeraim) planted in them are considered a karpef. If so, how can municipal eruvin include parks and gardens? Many reasons for leniency have been suggested:
- Parks planted for recreation are considered essential for living and are different than the Gemara’s case (Meiri, Maharsham, Nezirus Shimshon).
- Karpef is only when the wall (or eruv) was constructed first and then the seeds were planted (Divrei Malkiel, Sefer Poras Yosef).
- Seeds that grew by themselves (weeds) are permitted.
- Zeraim planted by non-Jews are permitted and cannot invalidate our eruv, plus it is no worse than if they grew by themselves (R.Shlomo Kluger, Divrei Malkiel).
- Does not apply to a city where the majority is urban and a small part has planted seeds (Dvar Shmuel, Chacham Tzvi). Moreover, it is part of the city planning and enhances it, so it wouldn’t be mevatel the dira aspect (Dvar Shmuel).
These leniencies are all subject to dispute, but over time the custom in many cities has been to be lenient based on some/all of them when better options (e.g., enclosing the area) are unavailable.