Aims. To investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry (e.g. in the context of exoplanet searches) and to quantify the expected effects in different regions of the HR-diagram. Methods. Stellar surface structures (spots, plages, granulation, non-radial oscillations) are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter. We use theoretical considerations supported by Monte Carlo simulations (using a starspot model) to derive statistical relations between the corresponding astrometric, photometric, and radial velocity effects. Based on these relations, the more easily observed photometric and radial velocity variations can be used to predict the expected size of the astrometric jitter. Also the third moment of the brightness distribution, interferometrically observable as closure phase, contains information about the astrometric jitter. Results. For most stellar types the astrometric jitter due to stellar surface structures is expected to be of the order of 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (< 0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of the order of 1 micro-AU, sufficient to allow the astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general a negligible impact on the detection of large (Jupiter-size) planets and on the determination of stellar parallax and proper motion. From the starspot model we also conclude that the commonly used spot filling factor is not the most relevant parameter for quantifying the spottiness in terms of the resulting astrometric, photometric and radial velocity variations.

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.

Education is increasingly being framed by a competence mindset; the value of knowledge lies much more in competence performativity and innovation than in simply knowing. Reaching such competency in areas such as astronomy and physics has long been known to be challenging. The movement from everyday conceptions of the world around us to a disciplinary interpretation is fraught with pitfalls and problems. Thus, what underpins the characteristics of the disciplinary trajectory to competence becomes an important educational consideration. In this article we report on a study involving what students and lecturers discern from the same disciplinary semiotic resource. We use this to propose an Anatomy of Disciplinary Discernment (ADD), a hierarchy of what is focused on and how it is interpreted in an appropriate, disciplinary manner, as an overarching fundamental aspect of disciplinary learning. Students and lecturers in astronomy and physics were asked to describe what they could discern from a video simulation of travel through our Galaxy and beyond. In all, 137 people from nine countries participated. The descriptions were analysed using a hermeneutic interpretive study approach. The analysis resulted in the formulation of five qualitatively different categories of discernment; the ADD, reflecting a view of participants’ competence levels. The ADD reveals four increasing levels of disciplinary discernment: Identification, Explanation, Appreciation, and Evaluation. This facilitates the identification of a clear relationship between educational level and the level of disciplinary discernment. The analytical outcomes of the study suggest how teachers of science, after using the ADD to assess the students disciplinary knowledge, may attain new insights into how to create more effective learning environments by explicitly crafting their teaching to support the crossing of boundaries in the ADD model.