Do you know anything about celestial navigation?

What's important in the software?
http://opencpn.org/ocpn/node/143

Other than sighting Polaris' angle from the horizon, and noting that the sun
rises in the east and sets in the west, I have never had a need for
celestial navigation software on Android.

However, I want to test out celestial navigation, on a lark mostly, but more
to learn how it's done (in case I ever need it in an emergency) and to show
a high school math teacher how its done so she can use it to make trig more
interesting to her students.

Looking up celestial navigation software, I find plenty of paywa
https://play.google.com/store/apps/d...id=com.vikrant
https://play.google.com/store/apps/d...ics.app.celnav
https://play.google.com/store/apps/d...otz.starstruck
http://appcrawlr.com/android/observa...celestial-body
etc.

But it's standard policy to never buy the payware until you've exhausted the
freeware, if for no other reason than you *know* exactly what you need the
payware to do if/when the freeware fails. (99% of the time or more, the
freeware does what you need, at least on Android.)

Looking up the freeware celestial navigation software, I find:
https://play.google.com/store/apps/d...stroNavigation
https://play.google.com/store/apps/d...Sightreduction
https://play.google.com/store/apps/d...stialnavigator

All I ask in this thread are the basic questions anyone new to celestial
navigation would ask:
a. What freeware is the best one to try out first?
b. What are the minimum required features of that freeware