Galilean shows the four bright Galilean moons of Jupiter -- Io, Europa, Ganymede and Callisto -- and their shadows, along with the position of Jupiter's "great red spot" (GRS).
To see it in action: Galilean on Shallowsky.com. Here's a screenshot:
This is a "progressive web app", meaning that you should be able to install it to the home screen of Android or iOS phones and tablets. It doesn't collect data or need any funky permissions, and shouldn't need to use the network at all once you have the images downloaded.