Installing. Android blocks apps from outside the Play Store by default. When you open the file, allow installation from your browser or file manager, then tap it again. To check what you already have, open Settings, Apps, WhenStars, and read the version at the bottom. If it does not match the number above, the install did not replace the old build.
It will not update over a Play Store install, and the Play Store will not update it. Google re-signs what it distributes, so this file and the store's copy count as two different apps to Android. If you already have WhenStars from the Play Store, uninstall it first, and expect to start with a fresh chart: uninstalling takes the app's data with it. Nothing here needs the Play Store to run.
Android 7.0 or newer, and it needs a connection: the readings are worked out
on whenstars.com. Everything is free.