Q. I live on the penthouse of a tall building and I can see the sea from far. How close does one have to be to the water, to be able to say tashlich?
A. On question 986 regarding the dateline and how far according to the Chazon Ish the waters close to the shore are considered like the land. we wrote:
Horav Shlomo Miller's Shlit”a opinion is that any island, vessel or individual found on the area of a Techum Shabbos (2000 amos or approximately one kilometre) from shore, should follow that same date. We find that Techum Shabbos creates an area of belonging and of being part of the city, in regards to the reading of the Megilah (Talmud Megilah 3b, Shulchan Aruch O.H. 688). The above also applies to the rulings, decrees and taxes imposed on the city (Shiltei Hagivorim – Bava Basra 11a on Ri”f, Maharam Alshich 59). Chaukei Chemed ( Gittin 4b .) applies the above to the acceptance of an early Shabbos time.
The Rov also considered the distance of sixteen mil (approximately sixteen kilometres) mentioned in regards to separating ma'aser from animals as one single distinct entity. The Talmud (Bechoros 54b.) explains all sheep herded in an area that is visible to the naked eye, are to be seen as one flock. Poskim disagree if this means an observer standing in the middle of a imaginary circle of a radius of eight mil creating a total area of a sixteen mil diameter (Tiferes Yisroel – Mishna ibid.- Chasdei Yehonoson) or actually it means that one can see as far as sixteen mil, and the area is double (Shvus Yaakov 3: 31 et. al.)
This amount of space applies also to other Halochos that depend on visual contact. Igrois Moshe (O.H. 3: 85) debates whether it applies to rending one's clothes when approaching Yerushalaim nowadays, for one that resides within that distance and is constantly in visual contact. Similarly, Betzel Hachochma (3: 40) depends this shiur for the brocho recited when seeing a kever or cemetery after thirty days.
Horav Shlomo Miller's shlit'a opinion is that within sixteen mil distance from the shore, it may be considered at least in doubt of belonging already to the date observed on shore. Therefore, if in the main landmass it is Shabbos, anyone within that distance, should be stringent in its observance."
Regarding how close does one have to be to the sea to see the water from far and be able to say tashlich, the Rov's opinion is also a sixteen mil distance.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit'a
Important Numbers
