977. Designating a Shabbos Base from a Distance
Eiruvin 7:3
The rule about designating a landmark for one’s Shabbos base (in the previous halacha) applies to a person without financial means, so that we do not burden him with the expense of leaving food for two meals, or to one who is far away, such as a person who is on a journey and is concerned that night will soon fall. This is so as long there would be enough time remaining in the day for him to reach the designated spot before nightfall if he ran as hard as he could, and there are 2,000 cubits (about 3,500 feet) or fewer between him and that place at nightfall. However, if he was neither far from his desired location nor lacking the means to make an eiruv of food, or if there was not enough time left in the day for him to reach his designated spot before nightfall by running as hard as he can, or if there were more than 2,000 cubits between him and his designated spot at nightfall, or if he did not specify the location he intended as his Shabbos base, in all of these cases one may not designate a distant spot as his Shabbos base. Rather, he is given only 2,000 cubits in each direction from the place he is standing at nightfall.
Eiruvin 7:4
If a person stood in a private domain before Shabbos began and he designated it as his Shabbos base, or if he was on a journey and had the intention to spend Shabbos in a private domain that he knew of, thereby designating it as his Shabbos base, then he may walk throughout that entire domain plus 2,000 cubits in every direction. However, if the private domain in question was not enclosed for use as a dwelling, or if it was a mound or a valley, then if it is the size need to plant two seah of seed (about 150’x75’) or smaller, he may traverse the entirety of that domain plus 2,000 cubits in every direction. If it is larger than the size needed to plant two seah, his spot is only four cubits in that domain (about 6’) from which he may proceed 2,000 cubits in every direction. The same rules apply should one place his eiruv in a domain that was not enclosed for use as a dwelling.