Long-distance observations: integrating atmospheric optics, meteorology, and citizen science.

Long-distance observations (LDOs)-the visual or photographic documentation of terrestrial objects from distances often exceeding the geometric horizon-are an emerging interdisciplinary field. However, a robust scientific basis that integrates empirical records with atmospheric physics is currently lacking. This study establishes such a methodology by combining a systematic typology of LDOs with a quantitative analysis of their controlling physical factors. The methodology integrates numerical simulations with analyses of photographic case studies from dedicated observer communities, including record-setting observations from Turkey (493 km), South America (484 km), and Central Europe (over 200 km). Analysis demonstrates that extreme visual range is primarily enabled by strong atmospheric refraction within thermal inversion layers. Key findings reveal the critical role of atmospheric transparency, turbulence, and illumination geometry in modulating observational success. It is concluded that LDOs serve a dual purpose: (i) as natural experiments for studying stratified atmospheric states and (ii) as a valuable, community-driven citizen-science practice that provides high-resolution spatio-temporal data complementary to formal meteorological monitoring.