About Solarc

What Solarc is

Solarc is a free web application that shows sunrise, sunset, twilight, golden hour and live sun position for any point on Earth. You can tap a location on the interactive map or search for a city, and read the entire solar day for that exact spot. No signup is needed and the application requires no API keys to use.

The site is live at https://www.solarc.app.

How the math works

Solar event times are computed with the astronomy-engine library, an MIT-licensed open-source astronomy library based on the same VSOP87 planetary theory used by NASA mission planning tools. Sun altitude and azimuth, sunrise and sunset, twilight boundaries and solar noon are all derived from the same primitive calls so they stay internally consistent.

Time zones are handled with Luxon and tz-lookup, which maps any latitude and longitude to the correct IANA timezone, including daylight saving transitions and political edge cases. Polar day and polar night are surfaced as explicit flags on the data contract rather than treated as a bug.

Accuracy and testing

The codebase has 325 automated tests across the solar core, the seasonal pages and the eclipse data. The regression suite verifies two things:

• Plausibility across 50 reference cities × 4 dates (March equinox, June solstice, September equinox, December solstice) totalling 200 sanity-check assertions on sunrise, sunset and solar noon ordering, day length bounds, and timezone correctness.
• NOAA anchors at five locations (Bucharest, Tokyo, Sydney, London and one more) on solstice dates, pinned to the NOAA Solar Calculator with a 3-minute tolerance to catch any silent drift.

Local terrain and atmospheric refraction can shift the visible sunrise and sunset by an additional minute or two beyond what any algorithm can predict from latitude, longitude and date alone. Solarc’s numbers are the geometric event in standard atmospheric conditions.

Sources and libraries

• astronomy-engine (MIT) for solar event computation and eclipse circumstances
• NOAA Solar Calculator as the cross-check reference
• Luxon for time zone arithmetic
• OpenFreeMap for vector map tiles, served without an API key
• MapLibre GL for the WebGL map rendering
• OpenStreetMap for the underlying geographic data that feeds OpenFreeMap
• GeoNames for the city population dataset used to generate the 500 static city pages

What is on the site

• The interactive map at / with sun overlays, daylight wedge and time slider
• 500 static city pages at /sun/[country]/[city] with a year overview, monthly tables and city-specific FAQs
• Programmatic intent pages for sunrise and sunset today and tomorrow, solar noon, and golden hour near you
• Seasonal pages for the summer solstice, winter solstice, equinox, daylight saving time and the next solar eclipse with per-city visibility

Privacy and license

Solarc does not require an account and stores no personal data on its own servers. Analytics and advertising scripts load only after you accept cookies. Full details are in the privacy policy. Content on the site is free for personal and commercial use.

Who maintains Solarc

Solarc is built and maintained by Mihai Popa, an independent developer. It is not affiliated with any large organisation. For corrections, bug reports or partnership ideas, the contact email is on the privacy page.

Frequently asked questions