- astropy: 
pip install astropy - matplotlib: 
pip install matplotlib - yaml: 
pip install PyYAML 
python sim.py
- Space: play/pause simulation
 - Enter: show/hide labels
 - t: show/hide legend
 - Up/Down: speedup/slow down simulation
 - u: switch display unit (UA or meter)
 - d: switch draw option (keep old positions)