

The smallest division of a calendar is the day: the periodĪfter which the Sun returns to about the same place in the sky. Halloween, your birthday, or the annual tax statement filing date. Weekend, the farmers' market, the harvest, the summer vacation, Humanity has used calendars for thousands of years, because calendarsĪre useful if you want to make appointments, or if you have to dateĪstronomical observations to discover important periodicities, andĪlso if you need to know when periodic things return, such as the a representation of a division of days, for example on paper.a method for grouping days into larger periods, for example weeks,.What are the best rules for leap years?.The structure of these pages is as follows: What did the calendar of Julius Caesar look like?.When is there going to be another Blue Moon?.Why does the year in the Gregorian calendar begin with January 1st and not with December 25th?.

Is the rotation of the Earth around its axis or around the Sun slowing down, and is that a problem for the calendar?.If you divide the year (orbital period) of a planet such as Mars into 12 months where each month corresponds to 30 degrees of motion around the Sun, and the year begins at the ascending equinox, then what is the length of each month?.

