 FACT: The Jewish Sabbath does not necessarily end when the sun goes  down, but when it gets dark which could be up to an hour later.  Also, an observant Jew would not be able to travel outside a city on  Sabbath, even on foot, for more than 2,000 biblical "cubits" (~1,000  yards) from the place he was in when Sabbath began. Therefore, Avram  would not have been willing to travel, leading his horse, on  Sabbath.