|apparent day||mean solar day||common day|
|sidereal day||tropical year||shake|
|millennium||figure out calendar re-use|
Introduction and Overview
Units of time have not been particularly fixed over the years because the primary gauge of time, earth's rotation and revolution, is not fixed. A mean (average) solar year today is 365.2422 days, but the earth's rotation is continually slowing. Every so often, the world's governments have to add a leap second to the master clocks, just to make up the extra time. Millions of years ago, the year was over 400 days long. Millions of years from now, it will be much shorter than it is now. At some point we will either have to change the length of the second or change the way we measure a day.
That is not the only concern. The earth's rotation and revolution are not constant. Occasionally it will speed up or slow down. These changes usually cancel each other out and we never notice them.
The main problem has come with defining a calendar that handles these changes and also takes care of the fact that a year is not an even number of days. The Gregorian calendar was designed to correct this. So now we have leap years, except in century years, excluding those century years divisible by 400 when we do have leap years. Another problem is that the year is not evenly divisible by a week, which is why we need a new calendar every year.
You can figure out when the current year's calendar will next be useful or which previous calendar will work for the current year by using the following table:
For the most comprehensive treatment of measurements, find "NTC's Encyclopedia of International Weights & Measures" by William D. Johnstone at your local library.
Note: Because of round-off errors, converting from very large units to very small units or vice-versa may not be accurate (or practical). Conversion factors can be found by converting a quantity of 1 unit to another unit several steps above or below the first. You may need to string several conversion factors together to find the factor from a very large unit to a very small unit, and then you can use a calculator with sufficient digits to find your answer.