Urban night sky is drastically lit up by a few decorative buildings: Natural experiments from Earth Hour

Urban night sky is drastically lit up by a few decorative buildings: Natural experiments from Earth Hour | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Urban night sky is drastically lit up by a few decorative buildings: Natural experiments from Earth Hour Chu Wing So, Chun Shing Jason Pun, Shengjie Liu, Sze Leung Cheung, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6241967/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 01 Jul, 2025 Read the published version in Scientific Reports → Version 1 posted 11 You are reading this latest preprint version Abstract Light pollution, an underappreciated environmental issue, has gained attention in recent years. While controlling light pollution requires sustained efforts, Earth Hour offers a unique natural experimental setting to assess temporary lights-out measures. Using photometric and spectroscopic sensors, we observed up to 50% night sky darkening during Earth Hour from 2011 to 2024 in Hong Kong, primarily as a result of a small but critical number of lights-out instances in central business districts, as evidenced by crowd-sourced photography records. Weekend nighttime activities in the city remained unaffected. The emission reductions mostly occurred in the 445–500, 500–540, and 615–650 nm spectral ranges---corresponding to peak emissions from LED billboard screens--- and in the 585–595 nm range, associated with metal halide floodlights used for facades and billboards. Our study identifies these sources as major contributors to urban light pollution. By combining multi-modal observations, we offer a comprehensive assessment of light pollution sources and the potential benefits of sustainable lighting practices in urban environments. This research highlights the importance of targeted light pollution mitigation efforts and provides critical insights for policymakers to enhance urban sustainability and human well-being. Earth and environmental sciences/Environmental sciences/Environmental impact Physical sciences/Astronomy and planetary science Full Text Additional Declarations No competing interests reported. Supplementary Files SupplementaryMaterialforNaturalexperimentsfromEarthHour17Mar2025.pdf Cite Share Download PDF Status: Published Journal Publication published 01 Jul, 2025 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 24 Apr, 2025 Reviews received at journal 07 Apr, 2025 Reviews received at journal 06 Apr, 2025 Reviewers agreed at journal 29 Mar, 2025 Reviewers agreed at journal 29 Mar, 2025 Reviewers agreed at journal 29 Mar, 2025 Reviewers invited by journal 29 Mar, 2025 Editor assigned by journal 29 Mar, 2025 Editor invited by journal 25 Mar, 2025 Submission checks completed at journal 24 Mar, 2025 First submitted to journal 17 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6241967","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":439693934,"identity":"2f2297fc-d499-4c71-8098-628d38fc42c1","order_by":0,"name":"Chu Wing So","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzElEQVRIiWNgGAWjYLCCDzAGY0MCA4MEG2EdjDNgGojWwsxDkhb59uZn0jY1Nnnm7M3PHzDuSEtskG5LwKvF4MwxM+mcY2nFlj3HDBsYz+QkNsgcO4Bfi0QO2+3chsOJG27kAB3WVpHYIJHegN9hM4BaLEFa7r8hUgvDDaAWRrAtPCAtQIdJpBFw2Jlj5j97jqUlbjiTZjgjsS3NuE0iLQG/w9qbHxv8qLFJ3HD88IMPH9uSZfsl0gzwOwwFgIwnIiJHwSgYBaNgFBACAA1jSvVww+NPAAAAAElFTkSuQmCC","orcid":"","institution":"University of Hong Kong","correspondingAuthor":true,"prefix":"","firstName":"Chu","middleName":"Wing","lastName":"So","suffix":""},{"id":439693935,"identity":"3e69b875-f7b4-453a-af0e-92f08de660dd","order_by":1,"name":"Chun Shing Jason Pun","email":"","orcid":"","institution":"University of Hong Kong","correspondingAuthor":false,"prefix":"","firstName":"Chun","middleName":"Shing Jason","lastName":"Pun","suffix":""},{"id":439693936,"identity":"170a51bc-7b19-4baf-a187-48563d99ad8a","order_by":2,"name":"Shengjie Liu","email":"","orcid":"","institution":"University of Southern California","correspondingAuthor":false,"prefix":"","firstName":"Shengjie","middleName":"","lastName":"Liu","suffix":""},{"id":439693937,"identity":"68743343-52ba-485a-aced-8fcba92d3f5d","order_by":3,"name":"Sze Leung Cheung","email":"","orcid":"","institution":"Ho Koon Nature Education cum Astronomical Centre (Sponsored by Sik Sik Yuen)","correspondingAuthor":false,"prefix":"","firstName":"Sze","middleName":"Leung","lastName":"Cheung","suffix":""},{"id":439693938,"identity":"4063a7b8-7fbf-49b8-be54-e277f5c66189","order_by":4,"name":"Ho Keung Kenneith Hui","email":"","orcid":"","institution":"Ho Koon Nature Education cum Astronomical Centre (Sponsored by Sik Sik Yuen)","correspondingAuthor":false,"prefix":"","firstName":"Ho","middleName":"Keung Kenneith","lastName":"Hui","suffix":""},{"id":439693939,"identity":"c9a78cae-f121-42d7-96fd-8c246b0e591d","order_by":5,"name":"Kelly Blumenthal","email":"","orcid":"","institution":"National Astronomical Observatory of Japan","correspondingAuthor":false,"prefix":"","firstName":"Kelly","middleName":"","lastName":"Blumenthal","suffix":""},{"id":439693940,"identity":"a9fc0492-1cde-47dc-ac4c-de4d2ccf7ec0","order_by":6,"name":"Constance E. 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While controlling light pollution requires sustained efforts, Earth Hour offers a unique natural experimental setting to assess temporary lights-out measures. Using photometric and spectroscopic sensors, we observed up to 50% night sky darkening during Earth Hour from 2011 to 2024 in Hong Kong, primarily as a result of a small but critical number of lights-out instances in central business districts, as evidenced by crowd-sourced photography records. Weekend nighttime activities in the city remained unaffected. The emission reductions mostly occurred in the 445–500, 500–540, and 615–650 nm spectral ranges---corresponding to peak emissions from LED billboard screens--- and in the 585–595 nm range, associated with metal halide floodlights used for facades and billboards. Our study identifies these sources as major contributors to urban light pollution. 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