Why We Built an Offline Moon Tool
Not every tool needs a server. The moon phase is a great example of something that should just work, anywhere, forever.
The phase is pure maths
The moon's phase at any moment follows from orbital mechanics, so there is no reason to fetch it from an API. By computing it in the browser we remove the things that usually go wrong: rate limits, outages, API keys, and the privacy cost of pinging a third party every time you check the moon.
Better for everyone
The result is a tool that loads fast, works on a plane or a campsite with no signal, and never tracks you. It also means we can show a live 30-day calendar instantly without dozens of network calls. The forecast is different — live weather genuinely needs a data source — which is why only the Forecast tool goes online, and we are upfront about that.
What "offline" actually requires
Nothing exotic: the page ships its own CSS, a small vendored copy of the astronomy formulas and a few kilobytes of script, with no font, icon set or framework pulled from a CDN at runtime. Once the browser has the page, every phase, date and calendar cell is recomputed from the device clock, so the tool is as correct next year as it is tonight.
The one thing it cannot do
An offline tool cannot tell you whether the sky is clear. Phase and moonrise are certain; cloud cover is not, and that is the part that still needs a live source. If you are planning around the moon, check the phase here and the sky with the Weather Forecast before you commit to a night out.
Try the moon tool — it even works offline.
Open the Moon Phase tool