On Dec. 20, the day length was 9 hours and 21 minutes. By Jan. 31, the day length increases to 10 hours and 9 minutes. Each year there is a winter and summer solstice as well as a fall and spring equinox.
Didn't winter already start?
It depends on who you ask. Remember the astronomical seasons are based on the Earth's relation to the sun during orbit. Meteorologically speaking, winter began on Dec. 1 because seasons are classified this way based on the annual temperature cycle. Only climatologists and meteorologists use this classification. It is still astronomical seasons that are widely observed.