I have seen some microcontroller-based astronomic clocks for application in street lighting, in Spain. They use a set of look-up tables (province selectable) with the sunrise/sunset times for all the days in the year.
The tables are generated using a PC where the astronomic formulae are implemented.

Benefits:
1) The microcontroler does not need to make complex computations (including float numbers).

Drawbacks:
1) The user does not introduce the exact latitude/longitude but a province code (each province table is computed for the mean latitude/longitude of the province). The errors are limited to about 1 minute for a small region.
2) The sunrise/sunset times change slightly from one year to another due to the complex Earth movements. The tables should be updated at least every 10 years.

Alfredo.