<P> The concept of an international date line in Jewish law is first mentioned by 12th - century decisors . But it was not until the introduction of improved transportation and communications systems in the 20th century that the question of an international date line truly became a question of practical Jewish law . </P> <P> As a practical matter, the conventional International Date Line--or another line in the Pacific Ocean close to it--serves as a de facto date line for purposes of Jewish law, at least in existing Jewish communities . For example, residents of the Jewish communities of Japan, New Zealand, Hawaii, and French Polynesia all observe Shabbat on local Saturday . However, there is not unanimity as to how Jewish law reaches that conclusion . For this reason, some authorities rule that certain aspects of Sabbath observance are required on Sunday (in Japan and New Zealand) or Friday (in Hawaii and French Polynesia) in addition to Saturday . Additionally, there are differences of opinion as to which day or days individual Jews traveling in the Pacific region away from established Jewish communities should observe Shabbat . </P> <P> For individuals crossing the date line, the change of calendar date influences some aspects of practice under Jewish law . Yet other aspects depend on an individual's experience of sunsets and sunrises to count days, notwithstanding the calendar date . </P> <P> The date line is a central factor in Umberto Eco's book The Island of the Day Before (1994), in which the protagonist finds himself on a becalmed ship, with an island close at hand on the other side of the IDL . Unable to swim, the protagonist indulges in increasingly confused speculation regarding the physical, metaphysical and religious import of the date line . </P>

Are the prime meridian and international date line the same