Does a sky map app need your location?

Yes, a sky map needs to know roughly where you are, because the sky depends on your position on Earth and the time. It does not need your exact street address, and you can keep location access limited to the moments you use the app.

I build Sky Map, so read my explanation with that in mind.

Why location matters for the sky

From Oslo the Big Dipper never sets; from Sydney it barely clears the horizon, and the Southern Cross is high instead. Latitude decides which part of the sky you can see at all, and longitude together with the time decides where everything sits right now. Without a position, an app can only guess.

The precision needed is modest. One degree of latitude is about 111 km, and moving that far shifts the sky by about one degree, roughly two Moon widths. For finding stars, knowing your city is plenty.

What else the app reads, and why

Sky Map’s product page says it uses the iPhone’s GPS, compass and gyroscope to work out where you are and which way the phone points, then draws the sky over the camera view. So the full AR view uses three things beyond location:

• Compass and motion sensors to match the drawing to the direction you point.
• Camera for the live picture behind the labels.

Choosing the iOS settings

Open Settings > Privacy & Security > Location Services, pick the app, and choose While Using the App. That shares location only when the app is open. If you prefer less detail, turn Precise Location off and check whether the labels still match the sky for you.

The core sky map works without internet, so you do not need data in a field or on a mountain. Sky Map is free to download, the AR view is free, and some features are paid. For the app’s current data practices, read the privacy details on its App Store page before you grant access.

Read next: when the sky map points the wrong way, what to look for in a sky map app and how to find constellations.