<p> Astronomy is becoming a forgotten science, which is evident by its relatively low enrollment figures compared to biology, chemistry, and physics. A portable inflatable planetarium brings relevance back to astronomy and offers support to students and educators by simulating realistic astronomical environments. This study sought to determine if learning is improved in an inflatable planetarium by adhering to the design principles of the cognitive theory of multimedia learning (CTML), specifically the coherence principle, in an authentic classroom. Two groups of 5^th grade students of similar ability were purposefully assigned using a 1-teacher-to-many-students format with mean lesson lengths of 34 minutes. The experimental group was differentiated with <i>seductive details</i>, defined as interesting but irrelevant facts that can distract learning. The control group (<i> n</i> = 28), with seductive details excluded, outperformed the experimental group (<i>n</i> = 28), validating the coherence principle and producing a Cohen's effect size of medium practical significance (<i>d</i> = 0.4). These findings suggest that CTML, when applied to planetarium instruction, does increase student learning and that seductive details do have a negative effect on learning. An adult training project was created to instruct educators on the benefits of CTML in astronomy education. This study leads to positive social change by highlighting astronomy education while providing educators with design principles of CTML in authentic settings to maximize learning, aid in the creation of digital media (astronomical simulations/instructional lessons for planetariums) and provide valuable training for owners of inflatable planetariums with the eventual goal of increasing student enrollment of astronomy courses at the local level.</p>

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