section_path: Systems/Solar
section_name: Solar
paragraph_text: Solar
  
  Solar calendars assign a date to each solar day. A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day. Other types of calendar may also use a solar day.
  
  The Egyptians appear to have been the first to develop a solar calendar, using as a fixed point the annual sunrise reappearance of the Dog Star— Sirius, or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River. They built a calendar with 365 days, divided into 12 months of 30 days each, with 5 extra days at the end of the year. However, they did not include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate.