How Stella decides if tonight is worth it
The Tonight score isn't a vibe — it's a handful of real measurements weighed into one decision. Here's exactly what goes in, where the data comes from, and what we won't fake.
One score, not ten browser tabs
Deciding whether to go out usually means cross-checking a weather app, a Moon calculator, a light-pollution map and an astronomy forecast — then guessing how they add up. Stella does that synthesis for you.
It weighs every signal below for your exact location and the hours you'd actually be observing, then returns a single Tonight score and the specific window when conditions are best. The goal is a decision you can make at a glance: go, or don't.
What the score is made of
These are the inputs, each evaluated for your spot and night rather than a regional average. No single one decides the night — clear skies under a full Moon still aren't a deep-sky night, and Stella accounts for that.
Where the data comes from
Every input traces back to a named, public source — not a proprietary black box. Weather and astronomy-grade cloud, transparency and seeing forecasts come from Open-Meteo. Space-weather and aurora data come from NOAA's Space Weather Prediction Center. Satellite and ISS pass predictions use orbital elements from Celestrak, and rocket-launch schedules from The Space Devs. Dark-sky parks, reserves and certified places come from DarkSky International's official registry.
Sun, Moon and twilight timing are computed with standard, well-validated astronomical algorithms — the same math observatories use — so the darkness and Moon windows are calculated, not guessed.
How we measure darkness (and where it's honest about precision)
A site's darkness is rated on the Bortle scale (1 = pristine wilderness sky, 9 = inner city). But not all Bortle numbers are equally trustworthy, so Stella labels how each one was derived instead of pretending they're all the same:
- Measured — from real sky-quality-meter readings (mag/arcsec²). Shown to one decimal, because it's earned.
- Modeled — derived from satellite artificial-brightness data. Also continuous, and flagged as modeled.
- Curated — taken from a site's certification survey or a published source.
- Estimated — sampled from NASA's Black Marble night-lights imagery where no better figure exists, and clearly marked '(est.)' so it's never mistaken for a measurement.
What we won't fake
Plenty of the sky is location-specific, and Stella refuses to invent it. It won't assert a planet's rise time or an eclipse's local coverage as a one-size-fits-all fact — those are computed for your actual coordinates, or pointed to the precise tool that does.
And the honest answer is sometimes "stay home." A forecast that says tonight isn't worth the drive is as valuable as one that says go — it saves you the two-hour round trip into clouds. Stella is built to tell you both, with the reasons behind the verdict.
See the verdict for your own sky — Stella does this math for your exact location, tonight.
Coming soon —Get early accessHow it works: FAQ
How is the Tonight score calculated?
Stella weighs cloud cover, atmospheric transparency, seeing (steadiness), moonlight, the astronomical-darkness window and forecast confidence for your exact location and the hours you'd observe, then combines them into one score plus the best window. No single factor decides it — clear skies under a bright Moon still aren't a deep-sky night.
Where does Stella's data come from?
Weather and astronomy forecasts from Open-Meteo, space weather and aurora from NOAA's Space Weather Prediction Center, satellite/ISS elements from Celestrak, launch schedules from The Space Devs, and dark-sky places from DarkSky International. Sun and Moon timing is computed with standard astronomical algorithms.
How is sky darkness (the Bortle scale) measured?
Where possible, from real sky-quality-meter readings in magnitudes per square arcsecond; otherwise modeled from satellite brightness, taken from certification surveys, or estimated from NASA night-lights imagery. Stella labels which method was used and only shows decimal precision when it's actually measured or modeled.
Why does Stella sometimes tell me not to go out?
Because a confident "don't bother" is as useful as a "go." If the Moon, clouds or transparency mean tonight won't deliver, saying so saves you a wasted trip — and Stella shows the reasons so you can decide for yourself.