 Varro used it in 37 BC to fix calendar dates for the start of the four seasons, which would have been impossible only 8 years earlier.A century later, when Pliny dated the winter solstice to 25 December because the sun entered the 8th degree of Capricorn on that date, this stability had become an ordinary fact of life.Although the approximation of days for the tropical year had been known for a long time ancient solar calendars had used less precise periods, resulting in gradual misalignment of the calendar with the seasons.