Barbara Navarrao age 14, of San Francisco’s a if,, for her question:

How is the year measured?

The calendar year is based on the earth's orbit around the sun. We call this period of time the solar year and it is estimated to be 365 days 5 hours 48 minutes and 45,375 seconds. As you can see, this does not divide into an exact number of calendar days. In a calendar year of 365 days there are 5 hours 48 minutes 45 seconds and part of a second left over. The earth on its orbit around the sun, it seems is not interested in making an orderly calendar.

We remedy the slight error by having leap years. Once every four years we add an extra day to the calendar to gather up those quarter days we lost on each of the 365 day calendars. True, our calendar is not yet perfect. But in 4,000 years it makes an error of only one day.