Available Data Sources

Library		Description
`sunrisesunsetlib`	^{Not Recommended}	Somewhat inaccurate and sometimes buggy. Does not support altitude, seconds-based calculation, solstice, equinox, or sun position. Based on “Almanac for Computers” by the USNO. github.com/mikereedell/sunrisesunsetlib-java
`ca.rmen.sunrisesunset`	^{Partially Recommended}	Similar to `sunrisesunsetlib` but with reasonable precision. Does not support altitude, solstice, equinox, or sun position. Based on the algorithms published by NOAA. github.com/caarmen/SunriseSunset
`time4a-simple`	^{Not Recommended}	Somewhat inaccurate. Does not support altitude. Based on “Almanac for Computers” by the USNO.
`time4a-noaa`	^{Partially Recommended}	Same algorithm used by `ca.rmen.sunrisesunset` with reasonable precision. Does not support altitude. Based on the algorithms published by NOAA.
`time4a-cc`	^Recommended	Good precision taking the altitude of locations into account. Based on “Calendrical Calculations” by Dershowitz/Reingold. Supports all features.
`time4a-time4j`	^{Recommended Default}	Best precision taking the altitude of locations, the elliptic shape of the earth and typical weather conditions into account. Based on “Astronomical Algorithm” by Jean Meeus. Supports all features. github.com/MenoData/Time4A

A few important details:

Do not expect precision better than minutes. The app hides seconds by default (but this can be enabled).
The precision of the USNO and NOAA algorithms tends to be very inaccurate in polar regions.
The time4j and cc algorithms may differ substantially (up to 10 minutes) from algorithms that do not account for altitude.
One difference between the time4j and cc algorithms is that cc only assumes the altitude of the observer by an approximated geodetic model, while time4j does it using a spheroid (WGS84) and the assumption of a standard atmosphere (for refraction).
None of these algorithms are able to account for local topology (a mountain directly in front of you), or deviating local weather conditions.