A month is measurement of time based on days and years. Each year is divided into twelve months. Each month has between 28 and 31 days. Table 1 shows the months in order and how many days are in each month.
|Number||Name||Number of days|
|2||February||28 or 29|
|Table 1 - months|
The number of days in February is 28 days for three years, then 29 days on leap year, except every 100 years when there is no leap year, except every 400 years when there is a leap year after all. The flow chart in illustration 2 shows how this works.
|Illustration 2 - Leap year flow chart.|
The following poem is a memnonic for remembering the number of days in each month:
Thirty days has September, April, June and November.
All the rest have thirty-one
Except February which has twenty-eight or twenty-nine.
People have been working on systems to measure the passage of days for many thousands of years. The solar year (the time it takes for the earth to rotate about the sun) and the calendar year usually were different. The calendar year was often several days shorter than the solar year. Because of this, in one year winter might start in November, but ten year later it could start in January, and ten years after than in March.
After many variations on calendars, the Gregorian calendar set the official length of a year at 365.245 days (365 days + 1/4 days - 1/100 days + 1/400 days). The system of leap years was used to keep the calendar year closely synchronized with the solar year. Today, scientist also use 'leap seconds' to keep the calendar year even more closely aligned with the solar year.
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