Isfahan is one of the significant cultural centers of Iran before and after Islam. However, Safavid period’s architecture and its political and cultural influence on modern Iranian society have somehow led to limited studies on pre-Safavid Isfahan. An example of this imbalance can be seen in the expression “Isfahan, Nisf of the world,” which likely originated before the Safavid era but is now predominantly used to illustrate the grandeur of Safavid Isfahan. On the other hand, Abbas Amanat has put forth a hypothesis suggesting that the designation of Isfahan as the “half of the world” originates from this city being designated as the Prime Meridian in the Jalali calendar system. There is no concrete evidence to support this hypothesis, but, this paper, by juxtaposing historical and astronomical evidence, attempts to show that even if Isfahan was not formally designated as the Prim meridian in Malikshahi Zīj, nevertheless its meridian was the axis upon which the Jalali radix date was calculated. Thus, aside from its political centrality, this city has, to some extent, attained a cosmological significance for a limited period of time.