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Julius
Ceasar's system would have been ideal if the year really was 365.25
days long. Yet, this is not the case. For many reasons most
people want seasonal events, such as spring, winter, to occur on the
same year after year. This is probably due our agrarian history.
For example, the first day of spring occurs on March 21 according the calender every year. However, we know that spring really starts when the Sun is at the vernal equinox, but the vernal equinox moves slowly against the background stars because of precession. For our calender we have defined a year to be equal to the time neeed for the Sun to return to the vernal equinox. This time period is called the tropical year and is equal to 365.2422 mean solar days or just slightly less than 365.25 mean solar days. Another way to measure the length of the year is with respect to the distant stars. This method requires measuring the time needed for the Sun to return to the same position with respect to the stars in the sky. This time period is equal to 365.2564 mean solar days, or just slightly greater than 365.25 days. The difference between the sideral year and the tropical year is about 20 minutes and 24 seconds. What does this mean for our calender? |
