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Ken Ward's Astrology Pages

Astrology: Time

• Leap Years
• The Days And Sidereal Time

Time is often thought to be a difficult concept to define simply or to say what it is. Time can be defined operationally.

These pages are not concerned with "what time is". However, they are concerned with time as a real "thing". While time can be defined as "what clocks measure", there are certain aspects of time that are not arbitrary. Hours, minutes and seconds may be arbitrary, however days and years are not.

The day is related to the time the sun takes to apparently revolve around the earth, and the year is related to the seasons. Civil clocks must reflect the day and civil calendars must reflect the seasons.

Leap Years

Our calendar making begain in 46 BC with Julius Caesar, and this system is called the Julian system. Every 4th year, a day was added to the calendar, to make a leap year. This system was accurate to 0.0078 days each year. Over a century, this amounts to an error of about 3/4 of a day. By the 16th Century, the error was such that the beginning of Spring shifted from March 23rd to March 11th.

To correct this, Pope Gregory XIII introduced the Gregorian system in 1582, which made Spring start on 21st of March and used a more sophisticated system of leap years. The Gregorian calendar year is, on average, 365.2425 days in length. It will take about 3,300 years before this calendar is out of step by one day. The Gregorian system is the system we use in our civil calendars.

In the civil calendar, the year must be a whole number of days (by convention). The real year is approximately 365¼ days. Every 4th year we add on a day to make up for the difference between the tropical year (real year) and the civil year. Every new century, if it is divisible by 400, an extra day is added.

The Days and Sidereal Time

While a civil day is 24 hours, a real day is slightly longer. It is approximately 4 minutes longer. A real day is measured by the stars and is called Sidereal Time. A sidereal year is therefore approximately 366¼ days.