Nachtlichter (“Night Lights”) is an app that assists citizen scientists in creating outdoor lighting inventories on unprecedented spatial scales. The app was co-created by a team of German citizen scientists together with researchers at the GFZ German Research Centre for Geosciences as part of the Helmholtz Association’s “Nachtlicht-BüHNE” project. While using the app, participants walk from one street corner to another, classifying each light they see according to 18 categories of lighting application, as well as 13 possible additional characteristics such as size and color. The group plans to use this data to answer two questions: first, what lighting applications are responsible for the light that we see from space? Second, how does the character of lighting change on an urban to rural gradient? During two campaigns in 2021 and 2022, citizen scientists classified over a quarter of a million light sources, covering a total area of about 22 square kilometers. The co-design team is currently working together to analyze and publish results based on this data. Although the main campaign of the project is now completed, you are welcome to try the app yourself by visiting https://lichter.nachtlicht-buehne.de/.