Why do we sometimes have a full moon and how do astronimers know on what dates will it occur?

Question: Why do we sometimes have a full moon and how do astronimers know on what dates will it occur?
- Keywords:
  moon,
  phase,
  predict
Asked by to Nick, Chris on 19 Mar 2014. This question was also asked by .
- Nick Goldman answered on 19 Mar 2014:
  
  It’s caused by the combination of where the moon is in its orbit around the Earth (which takes about 4 weeks) and where the Earth is in its orbit around the Sun (which takes a year). Every 4 weeks, the moon is lined-up so that the part of it that is lit up by the sun is totally facing the Earth, and then we see a full moon.

Question: Why do we sometimes have a full moon and how do astronimers know on what dates will it occur?