The nineteenth century saw significant shifts in the cognitive life of Americans. As transnational critics have suggested, the energies of imperialism and the contemporaneity of different claims on unsettled land engendered a radically unsettled perception of space, one that permeated into the fabric of day-to-day life. Meanwhile, the co-presence of different orders of time, as Lloyd Pratt and Dana Luciano have argued, undercut their attempts to articulate supralocal categories of affiliation, whether nation, race, or gender. Both national time and transnational space were, then, in a state of contestation, tending towards heterogeneity. As yet critics have not thought about whether the dynamics of transnationalism might permeate into formulations of time; conversely, little analysis has been given to the role of time in transnational discourse. This article aims to begin to fill this lacuna through an extensive reading of antebellum invocations of "simultaneity-across-time." Through rejecting the national framework that supported Benedict Anderson's use of this term, I demonstrate that antebellum Americans understood simultaneity in explicitly transnational terms. The first section argues that as globalization wrought significant changes in the composition of the globe, connecting US citizens, like Richard Henry Dana Jr., and European scientists, like Alexander von Humboldt, to others far distant, conceptions of simultaneity became equally extended, stretching out across oceans. But, as my second section, focused on Edgar Allan Poe's Eureka, suggests, rather than tending to a world in which a single co-extensive time was shared by all, this process tended toward heterogeneity, with spots of temporal locality warping the linear passing of the antebellum clock. There was, then, no absolute sense of a shared, planetary time, instead a deeply-felt terror at a sudden interconnectedness of the planet.