Citizen science illuminates the nature of city lights

Summary

The Nachtlichter app was developed within a project called Nachtlicht-BüHNE (Citizen-Helmholtz Network for research on night light phenomena)5, using a co-design process in which academic and citizen scientists met regularly over a several year period. Our co-design process, app methodology, site selection, systematic variability of the observations, data pre-processing and data structure have already been described in detail5. This section therefore briefly covers the data and validation and focuses mainly on the methods unique to the analyses presented here.Nachtlichter data and validationIn a Nachtlichter observation, participants conducted a ‘survey’ while walking along a ‘transect’, which typically extended from one street corner to the next. The participants used the app to classify and count all of the light sources that they could see. A total of 18 light categories were used for the 2021 experiment (Fig. 2). Depending on the light type selected, participants provided additional information about the size, emission direction (that is, shielding), color and subjective brightness. Transects were pre-defined in most cases and selected and arranged to completely survey the publicly accessible areas covered by a reprojected DNB satellite pixel. We therefore somewhat undercount the total number of installed lights because we did not record lights installed in areas not visible from public spaces (for example, backyards, courtyards and rooftops; Supplementary Fig. 5).The observation time (of night) was not constrained, but the main experiment took place from 23 August to 14 November 2021, usually over a period of weeks for each pixel51. Additional smaller data-taking campaigns were conducted in the spring and autumn of 2022 to develop a correction for certain lighting types that were found to frequently turn off. The campaign in autumn of 2022 took place immediately after a German law requiring switch offs of some signs was passed22. However, as our statistics were...