Figure 1. Different extinction angles for Orion’s stars (stacked screengrabs from Stellarium [2021]). Title: How visible are celestial objects? Author: Victor Reijs (citizen scientist, Lochem, The Netherlands) Abstract This paper reviews models to predict or postdict naked-eye visibility of celestial objects. Several models are discussed, such as observational-based models (like Kolev’s and Schoch’) and combined mathematical and observational models (like Inklaar’s and Schaefer’s). After some observational definitions, a detailed sensitivity analysis of the author’s implementation of one combined (Schaefer’s) model is provided, identifying the most critical celestial, geographic, atmospheric, and observer parameters. To benchmark Schaefer's model, comparisons with real observations and other models (such as de Jong, Kolev, Ptolemy, Schoch) is provided. The author concludes with a summary of his findings, including recommendations for future study areas. Chapter 1: Introduction This paper discusses several models for determining the visibility of celestial objects using the naked eye. Beside the obstruction of the skyline, the solar glare can make a celestial object invisible, and this depends on celestial coordinates, geographic, atmospheric, and observer parameters. Before comparing models, this paper presents some definitions of relevant celestial bodies' altitudes, their visibility during night times and heliacal events. Three different visibility models are introduced: an observational model; a mathematical model; and a combination of both. The combined models are based on underlying atmospheric and physiological findings embedded in empirical/observational formulae. The author made some enhancements to one combined model: Schaefer’s model. A sensitivity analysis of the involved parameters identifies which parameters are essential and which ones can be ignored. Chapter 2: Definitions used to define the visibility of celestial objects. This chapter provides several definitions on angles, the extinction of an object during full darkness, and the heliacal events for stars; planets; and the Moon.