Who needs 3D when the universe is flat?

An overlooked feature in astronomy education is the need for students to learn to extrapolate three-dimensionality and the challenges that this may involve. Discerning critical features in the night sky that are embedded in dimensionality is a long-term learning process. Several articles have addressed the usefulness of three-dimensional (3D) simulations in astronomy education, but they have neither addressed what students discern nor the nature of that discernment. A Web-based questionnaire was designed using links to video clips drawn from a simulation video of travel through our galaxy and beyond. The questionnaire was completed by 137 participants from nine countries across a broad span of astronomy education. The descriptions provided by the participants were analyzed using hermeneutics in combination with a constant comparative approach to formulate six categories of discernment in relation to multidimensionality. These results are used to make the case that the ability to extrapolate three-dimensionality calls for the creation of meaningful motion parallax experiences.