This week I spoke about Techum Shabbos.
There are three shittos regarding whether Techum Shabbos is m’doreisa or m’d’rabanan.
1) The Rambam in Sefer Hamitzvos learns that even the techum of 2000 amos is Min Ha Torah.
2) The Rambam in Mishna Torah (Shabbos Perek 27) is chozer and says that only the techum of 12 mil is Min HaTorah. The techum of 2000 amos is only m’d’rabanan. The m’kor for this seems to be a Yerushalmi. This is the opinion of the Rif (end of Eiruvin Perek 1 )
3) The Ramban both in Sefer Hamitzvos as well as in Eiruvin (Perek 1 17b)writes that only Rabbi Akiva holds Techum of 2000 amos is Min HaTorah and only the Yerushalmi holds 12 mil is Min Ha Torah. The chachamim in the Bavli holds it is m’d’rabanan.
There is a gemara in Eiruvin (17b) which indicates one gets malkos for techum shabbos. The Ramban says this is going according to Rabbi Akiva who holds techum Shabbos is Min HaTorah
The Divrei Yechezkel (siman 7) has an interesting chakirah. How does one view the geder of Techum Shabbos. Is it a din that I am not allowed to leave my mokom (however your mokom is defined) and once I leave it I have violated the techum . Or do we say the issur is in walking 12 mil and the 12 mil is a shiur in the issur halicha.
This second tzad needs a little hesber because it is definitely not assur to walk 12 mil within the techum. Furthermore, if a walk a little bit outside my techum even if it is not 12 mil I am also chayav. That being said it seems the Ramban in Eiruvin 43a seems to hold like this tzad. The Avi Ezri (Shabbos Perek 27) explains the Ramban with this mehalech.
One nafka mina in this chakirah is chatzi shiur.Why is there no issur chayzi shiur for walking less than 12 mil. The Divrei Yechezkel says this shows the 1st tzad makes sense. Since techum Shabbos is a din in leaving your mokom, it is not shayach to say chatzi shiur. I saw Rav Sheinberg in his Mishmeres HaChaim discusses this chakirah with regard to ma'avir 4 amos. He says there is no chatzi shiur by 4 amos because by hotza’ah that each amah is not a davar chashuv in of itself – it’s only the sum which creates the chashivus.
L'chorah you can say the same vort by techum Shabbos even according to teh Ramban.
There are other ways to answer teh question. Maybe we don't say chatzi shiur by issurei melacha. Ayin in Mishmeres Chaim. You would then need to figure out if techum Shabbos is an issur melacha or a diff issur of Shabbos (like Shevisas Beheima). Rav Shach and the Divrei Yechezkel both discuss this as well.