Transparency is the foundation of everything we do. If you are going to trust us to tell you about the sky, you need to know how we work. This page documents our data sources, our verification processes, our update schedule, and our limitations. We believe honest observation starts with honest methodology.
Weather Data — Open-Meteo API
The current weather conditions displayed on our website come from the Open-Meteo API, a free, open-source weather API that provides global forecast data without requiring an API key. Open-Meteo aggregates data from multiple sources, including the Japan Meteorological Agency (JMA), the European Centre for Medium-Range Weather Forecasts (ECMWF), and the Global Forecast System (GFS). We query the API on page load for each of our five locations, requesting the following parameters: current cloud cover percentage, temperature at 2 meters, relative humidity at 2 meters, weather code, and daily values for maximum temperature, minimum temperature, sunrise time, sunset time, and daylight duration.
The data is updated on the API side every hour. We do not cache weather data on our server — your browser fetches it directly from Open-Meteo when you load the page. This means the data you see is always fresh, but it also means the display depends on your internet connection and Open-Meteo's availability. If the API is unreachable, we display "Updating..." rather than showing stale or fabricated numbers. We never make up weather data. Ever.
Open-Meteo's cloud cover data comes from satellite observations and atmospheric models, not from ground-based sky cameras. This means it represents the cloud cover over a broad area (approximately 11x11 kilometers for the ECMWF model) rather than the exact patch of sky above your head. A location might show 20% cloud cover while a single large cumulus happens to be directly overhead, or 80% cover while a lucky gap gives you a clear view. The data is representative, not literal. Use it as a guide, not a guarantee.
Visual Observation Verification
Every claim we make about atmospheric phenomena is verified by at least one team member observing the event directly. We do not publish secondhand reports, social media reposts, or unverified sightings. Hana Suzuki conducts the primary visual observations from our five fixed locations. David Park corroborates astronomical events with telescope observations and sky quality meter readings. Yuki Mori cross-checks all meteorological claims against JMA data and scientific literature.
Our observation protocol is standardized. For cloud type identification, we use the World Meteorological Organization's International Cloud Atlas as our reference. Each cloud type sighting is photographed, timestamped, and cross-referenced with satellite imagery from the Himawari-8 geostationary weather satellite. For astronomical events, we use Stellarium Plus for trajectory prediction and cross-check with NASA's Horizons ephemeris system. For light pollution measurements, David uses a Unihedron Sky Quality Meter-LU, calibrated annually against the manufacturer's standard.
We maintain an observation log that is available upon request. The log includes date, time, location, observer, phenomenon observed, equipment used, weather conditions, and photographic evidence where available. We do not publish the raw log publicly because it contains location data that could compromise the privacy of our field station. But we will share relevant entries with researchers, journalists, or serious sky watchers who contact us with a legitimate reason.
Sunset and Sunrise Times
Sunset and sunrise times displayed on our website are calculated by the Open-Meteo API using the NOAA solar position algorithm. This algorithm accounts for atmospheric refraction (the bending of light by Earth's atmosphere, which makes the sun appear slightly higher than it actually is) and the equation of time (the difference between clock time and solar time due to Earth's elliptical orbit). The times are given for each location's specific latitude and longitude, not a generic "Tokyo" coordinate.
The NOAA algorithm has an accuracy of approximately plus or minus one minute for locations at Tokyo's latitude. Actual sunrise and sunset may vary slightly due to local topography — a mountain to your west will make sunset earlier at ground level than the calculated time for your latitude. Our Odaiba and Mt. Takao sunset times are the most accurate because those locations have clear horizons. Shinjuku times are nominal because buildings block the actual horizon.
Golden Hour Calculations
We define golden hour as the period when the sun is between the horizon and 6 degrees above the horizon, and the solar elevation is decreasing (evening golden hour). This is the standard definition used in photography. The duration varies by season: approximately 48 minutes at the winter solstice, 72 minutes at the summer solstice. We calculate these durations using the solar position algorithm and update them seasonally. The exact golden hour for any given day depends on the sun's declination, which changes daily.
Light Pollution Mapping
David Park's light pollution survey was conducted between October 2022 and March 2023 using a Unihedron Sky Quality Meter-LU, which measures sky brightness in magnitudes per square arcsecond (MPSAS). Measurements were taken at 2 AM on clear, moonless nights to minimize interference from artificial light sources and lunar illumination. The survey covered all 23 special wards of Tokyo plus selected locations in the western suburbs and on Mt. Takao.
Each measurement location was selected to be representative of its neighborhood — not directly under a streetlight, not in a completely unlit alley, but in a typical outdoor position that an observer might actually use for stargazing. Measurements were taken with the meter pointing at the zenith (straight up) and held at chest height. Three readings were taken at each location and averaged. The Bortle scale classifications were derived from the MPSAS readings using the standard conversion table published by the International Dark-Sky Association.
The resulting map has a spatial resolution of approximately one measurement per 10 square kilometers in the densest wards, increasing to one per 2 square kilometers in the outer wards and western suburbs. This is not sufficient to capture block-level variations, but it is adequate for neighborhood-level guidance. We plan to resurvey the city in 2026 to account for changes in LED street lighting and new development.
Update Schedule
Weather data: Updated hourly via the Open-Meteo API, fetched fresh on every page load. Sky view profiles: Reviewed quarterly, updated when significant changes occur (new construction, park renovations, etc.). Sunset spot guides: Updated seasonally to reflect changing sunset azimuths. Cloud atlas: Updated when we observe new or rare phenomena. Stargazing guide: Updated monthly during peak observing seasons, quarterly otherwise. Light pollution map: Major resurvey planned every 3 years, with minor updates as needed.
Limitations and Caveats
We are not meteorologists. We are observers with scientific training and a commitment to accuracy, but we do not have access to the resources of a national meteorological service. Our observations are localized to Tokyo and may not apply to other cities or climates. Our weather data comes from models, not direct measurement at each location. Our light pollution map is a snapshot in time and does not account for temporary changes such as festival lighting, construction lighting, or power outages.
We make mistakes. When we do, we correct them promptly and document the correction in our fix log. We welcome corrections from readers. If you see something wrong, tell us. We will verify it and fix it. That is how science works. That is how trust works.