About the Gregorian Calendar This calendar system uses even more complex rules to determine when a leap day is to be added.

With an error of only about 2 seconds per year or 1 day in 31,250, it is roughly 10 times more accurate than today's Gregorian calendar and one of the most accurate calendar systems ever devised.

In general, any given year "n BC" becomes "-(n-1)" in the astronomical year numbering system.

Historians should take care to note the numerical difference of one year between "BC" dates and astronomical dates.

The ancient Egyptians originally employed a calendar based upon the Moon, and, like many peoples throughout the world, they regulated their lunar calendar by means of the guidance of a sidereal calendar.

They used the seasonal appearance of the star Sirius (Sothis);…

The Egyptian month began with the new moon—reckoned from the first morning after the waning crescent had become invisible—and was named after the major festival celebrated within it.

Since the lunar calendar was 10 or 11 days shorter than the solar year, a 13th month (called Thoth) was Sirius), when it could be observed on the eastern horizon just before dawn in midsummer; the timing of this observation would determine whether or not the intercalary month would be employed.

The "astronomical" dating system refers to an alternative method of numbering years.

It includes the year "0" and eliminates the need for any prefixes or suffixes by attributing the arithmetic sign to the date.

And this increased concentration has consequences: it strongly supports the idea that a Michael was a science writer for the Idaho National Laboratory and has been an intern at Wired.com, The Salinas Californian newspaper, and the SLAC National Accelerator Laboratory. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz.

He has also worked as a herpetologist and wildlife biologist. To find out what his latest project is, you can follow Mike on Google .

Calendars that are designed to reflect time spans other than the tropical year are not listed.