Privacy Policy

Twilyte doesn't track you. There's no account, no advertising, no analytics software, and no profile built about you or your device. This page explains, in full, everything the app does with data — because that list is short enough to just say plainly.

Location

Twilyte uses your device's location (with your permission, via your browser's or operating system's location prompt) to calculate the Sun, Moon, and star positions, twilight times, and light-pollution estimate for where you are. Your coordinates are used as follows:

• On your device: all astronomy calculations run locally in the app. Your position is never sent to a Twilyte server, with one exception that you choose: if you turn on clear-sky alerts, the approximate place they're for is kept so the alert can be worked out (see Clear-sky alerts).
• To Open-Meteo (open-meteo.com), a weather-forecast provider: your coordinates, and two points about 150 km from them toward the sunset and the sunrise, are sent to fetch the cloud forecast used for the "Clear & Dark" observing score and the sunset and sunrise colour, and to search for places by name. Open-Meteo does not require an account or API key and is not used for any purpose beyond serving that request.

To say when the International Space Station passes over, Twilyte also fetches the station's published orbit from CelesTrak (celestrak.org), at most every 12 hours. That request is the same for everyone: it contains no location or anything else about you, and the passes are worked out on your device. Like any web request it reaches CelesTrak from your IP address.

For the northern and southern lights it fetches the geomagnetic forecast published by the US National Oceanic and Atmospheric Administration's Space Weather Prediction Center (swpc.noaa.gov), at most every 3 hours. That request is also the same for everyone and contains nothing about you; whether the aurora could reach your sky is worked out on your device.

You can deny location access entirely and still use Twilyte by entering a ZIP code, searching for a place by name, or typing coordinates directly.

Motion sensors and camera

Two features in the Stars tab can use hardware beyond location. Both are optional, both ask before they start, and neither sends anything anywhere.

• Compass and motion sensors power Aim Assist and Sky View, which work out which way your phone is pointed so Twilyte can show you where a star is. The readings are used as they arrive and are not stored or transmitted. The one thing that is saved is a manual compass correction, if you set one — a single number, kept on your device.
• The camera is off by default. If you switch it on in Sky View, the live view from your rear camera is drawn behind the star overlay so you can line the two up. Nothing is photographed, recorded, saved, or uploaded — the image is displayed and discarded, frame by frame. Turning the camera off, or leaving Sky View, stops it.

Your calendar

In the iPhone app, "Add to calendar" opens iOS's own event editor with the times filled in, and nothing is added unless you tap Add. Exporting a whole month asks first, then asks iOS for permission to add events; that permission lets Twilyte add to your calendar but never read it. Nothing about your calendar is sent anywhere. On the website these buttons download a calendar file instead.

What's stored on your device

Your preferences and your last-used location are saved locally in your browser's or app's storage, so Twilyte remembers them between visits. None of this is transmitted anywhere — it never leaves your device except as described above.

• Your saved and current location
• Display preferences (12/24-hour time, red-light night mode, sky-brightness setting)
• Your Aim Assist compass correction, if you've set one
• The space station's orbit, for up to a week, so passes can be shown offline
• The geomagnetic forecast, for up to two days, for the aurora
• A short-lived cache (about an hour) of the weather forecast, to avoid re-fetching it constantly
• Whether you've dismissed onboarding messages, and until when the “Add to your home screen” suggestion is put off
• The last three days this browser opened Twilyte (dates only), so that suggestion waits for a second visit
• Whether clear-sky alerts are on, and the rounded place they're for
• When this browser was last counted by the visit counter, and the last total it showed, so that coming back later the same day isn't counted as another visit

You can clear all of this at any time by clearing your browser's site data for twilyte.info, or by deleting the app.

Anonymous visit counter

Twilyte shows a running count of visits on the Console tab. Loading the page pings a small counting service at most once a day; reloading the page, or leaving and coming back later the same day, doesn't ping it again. This is on the website only: the iOS and Android apps don't show the count and never contact the counting service. The website sends it nothing beyond what any web request inherently includes — not your location, not a device identifier.

Like any web server, the counting service receives your IP address with that request. So that it doesn't count the same visitor twice in one day, it keeps a one-way, salted hash of the address — never the address itself — for 24 hours, after which it's deleted automatically. For an IPv6 address only the first half, the part that identifies your network rather than your device, goes into the hash. The hash isn't linked to your location or to anything else, and is used for nothing but the count.

Clear-sky alerts (website only, optional)

On the website you can ask Twilyte to tell you when tonight looks clear and dark. It's off unless you turn it on, and your browser asks your permission to show notifications first. The iOS and Android apps don't offer it.

When you turn it on, a small Twilyte service (a Cloudflare Worker) keeps:

• the push address your browser gives it, which only lets it send a notification to that browser through the browser maker's push service (Google, Mozilla, Apple or Microsoft). The notification's text is encrypted so that service can't read it.
• the place the alerts are for, rounded to about 11 km (0.1°), its time zone, and whether you use 12- or 24-hour time
• the date of the last alert it sent, so you get at most one a day

Every afternoon it fetches the cloud forecast for that rounded place from Open-Meteo and works out tonight's score the same way the app does. No name, email or other identifier is involved. Turning alerts off deletes all of it straight away. If the push service reports that the address is no longer in use (for example, after you clear this site's data), it's deleted then.

In-app purchase (not offered yet)

The current iOS and Android apps sell nothing and never contact a purchase service: the week planner is free for everyone. A later version may offer it as an optional one-time purchase, and this page will say so before that version ships.

If it does, purchases will be processed entirely by Apple's App Store or Google Play, and Twilyte will never receive your payment details. To verify and restore a purchase across devices, the app will use RevenueCat, which is given an anonymous, random identifier for your installation (not your name, email, or Apple/Google account) along with the purchase receipt Apple or Google already issued. See RevenueCat's privacy policy for how they handle that data.

What Twilyte does not do

• No account, sign-in, name, or email is ever collected
• No advertising SDKs and no ad tracking
• No analytics software (no Google Analytics, no crash reporters, no session recording)
• No cookies — the app uses only local on-device storage described above
• No data is sold, shared, or used for any purpose beyond making the app work

Third-party services this app calls

Children's privacy

Twilyte is a general-audience reference tool and does not knowingly collect personal information from anyone, including children. Since the app collects no personal information from any user, no age-specific handling is needed.

Changes to this policy

If what Twilyte collects or how it's used changes, this page will be updated and the "last updated" date above will change accordingly.

Contact

Questions about this policy or the app can be sent via the GitHub repository. The Help page says how.