Cs-133 produces a microwave spectral line of 9,192,631,770 cycles per second. Since the day is defined as being divided into 24 hours of 60 minutes, each of 60 seconds, a day is defined as the length of time required for Cs-133 to produce 9,192,631,770 cycles 86,400 consecutive times. The calendar year is always expressed in whole days so the sun will rise in the morning, reach apex near midday, and set in the evening. A solar year or tropical year is defined as the number of days it takes for the Earth to complete one cycle around the sun. This is usually measured in terms of one of four solar events .
Short years occur when the calendar year is shorter than the solar year .
When this occurs, the date of a solar events in one year arrives before the solar event occurs in successive years. Phrased another way, the solar events occur on a later date on the calendar in successive years. Long years occur when the calendar year is longer than the solar year . When this occurs, the date of a solar events in one year arrives after the solar event occurs in successive years. Phrased another way, the solar events occur on an earlier date on the calendar in successive years.