The ancient Egyptians used a calendar with 12 months of 30 days each, for a total of 360 days per year. About 4000 B.C. they added five extra days at the end of every year to bring it more into line with the solar year. These five days became a festival because it was thought to be unlucky to work during that time.
The Egyptians had calculated that the solar year was actually closer to 365-1/4 days, but instead of having a single leap day every four years to account for the fractional day (the way we do now), they let the one-quarter day accumulate. After 1,460 solar years, or four periods of 365 years, 1,461 Egyptian years had passed. This means that as the years passed, the Egyptian months fell out of sync with the seasons, so that the summer months eventually fell during winter. Only once every 1,460 years did their calendar year coincide precisely with the solar year.
In addition to the civic calendar, the Egyptians also had a religious calendar that was based on the 29-1/2-day lunar cycle and was more closely linked with agricultural cycles and the movements of the stars.
During antiquity the lunar calendar that best approximated a solar-year calendar was based on a 19-year period, with 7 of these 19 years having 13 months. In all, the period contained 235 months. Still using the lunation value of 29-1/2 days, this made a total of 6,932-1/2 days, while 19 solar years added up to 6,939.7 days, a difference of just one week per period and about five weeks per century.
Even the 19-year period required adjustment, but it became the basis of the calendars of the ancient Chinese, Babylonians, Greeks, and Jews. The Arabs also used this same calendar, but Muhammad later forbade shifting from 12 months to 13 months, so that the Islamic calendar now has a lunar year of about 354 days. As a result, the months of the Islamic calendar, as well as the Islamic religious festivals, migrate through all the seasons of the year.
The Roman Calendar
When Rome emerged as a world power, the difficulties of making a calendar were well known, but the Romans complicated their lives because of their superstition that even numbers were unlucky. Hence their months were 29 or 31 days long, with the exception of February, which had 28 days. However, four months of 31 days, seven months of 29 days, and one month of 28 days added up to only 355 days. Therefore the Romans invented an extra month called Mercedonius of 22 or 23 days. It was added every second year.
Even with Mercedonius, the Roman calendar eventually became so far off that Julius Caesar, advised by the astronomer Sosigenes, ordered a sweeping reform. 46 B.C. was made 445 days long by imperial decree, bringing the calendar back in step with the seasons. Then the solar year (with the value of 365 days and 6 hours) was made the basis of the calendar. The months were 30 or 31 days in length, and to take care of the 6 hours, every fourth year was made a 366-day year. Moreover, Caesar decreed the year began with the first of January, not with the vernal equinox in late March.
This calendar was named the Julian calendar, after Julius Caesar, and it continues to be used by Eastern Orthodox churches for holiday calculations to this day. However, despite the correction, the Julian calendar is still 11-1/2 minutes longer than the actual solar year, and after a number of centuries, even 11-1/2 minutes adds up.
The Gregorian Reform
By the 15th century the Julian calendar had drifted behind the solar calendar by about a week, so that the vernal equinox was falling around March 12 instead of around March 20. Pope Sixtus IV (who reigned from 1471 to 1484) decided that another reform was needed and called the German astronomer Regiomontanus to Rome to advise him. Regiomontanus arrived in 1475, but unfortunately he died shortly afterward, and the pope's plans for reform died with him.
Then in 1545, the Council of Trent authorized Pope Paul III to reform the calendar once more. Father Christopher Clavius, S.J, did most of the mathematical and astronomical work. The immediate correction advised by Father Clavius and ordered by Pope Gregory XIII, was that Thursday, Oct. 4, 1582, was to be the last day of the Julian calendar. The next day would be Friday, Oct. 15. For long-range accuracy, a formula suggested by the Vatican librarian Aloysius Giglio was adopted: every fourth year is a leap year unless it is a century year like 1700 or 1800. Century years can be leap years only when they are divisible by 400 (e.g., 1600 and 2000). This rule eliminates three leap years in four centuries, making the calendar sufficiently accurate.
In spite of the revised leap year rule, an average calendar year is still about 26 seconds longer than the Earth's orbital period. But this discrepancy will need 3,323 years to build up to a single day.
Source: Source: http://www.infoplease.com/ipa/A0002061.html
The History of Paper...
Just imagine Ts'ai Lun. The Chinese government official and scholar is grinding up plants - mulberry bark, linen and hemp. He makes a big wet mush of separate fibers, and then spreads it all out in a mat made of coarse cloth and a bamboo frame.
It looks like he's got a mess on his hands, and chances are his family, friends and neighbors are making fun of him. But when he's done, and the sun has dried the matted material, he's made something really remarkable.
Ts'ai Lun, 2,000 years ago, has made paper, and it will become one of the most important inventions ever.
Even though archaeological evidence shows that paper may have been made even a little earlier, Ts'ai Lun was the first to have his efforts recorded. Like many inventors through the centuries, he built upon the work of others.
Okay, people had written even before paper was invented. They scratched on cave walls, painted too, and drew characters on wet clay. They even wrote on papyrus made from thinly sliced papyrus reed, which they glued together to make a sheet.
People did the weaving to make papyrus. What Ts'ai Lun and others discovered was that plant fibers, separated and suspended in water, would form their own woven mats: paper.
Chinese paper-making spread slowly but steadily all over the world, from Asia into Africa and Europe. Soon just about everyone knew how to make paper. More books were printed, people became better educated, and these better-educated people scratched their heads, trying to figure out a substance that might provide even more paper-making material.
One of those people was a man named Rene de Réaumur who, in the 1700s, watched a species of wasp we now call the paper wasp. These insects were munching on wood. Not eating it, exactly, but chewing it up, spitting the mush back out and forming nests with it. It seemed to him that the wasps were making paper out of wood.
We still follow Réaumur's advice and the wasps' example, although paper-making has become a more complex and efficient process, and its products incredibly varied and advanced.
People picked up the paper challenge. One person, a man named Kellar, learned how to grind wood efficiently. Others invented new ways to separate wood fibers. If Réaumur had written down his paper recipe - or more accurately, the wasps' recipe - it might have looked like this: wood fiber + water + energy = paper.
We still make paper using that same basic formula. We just vary the kinds of wood fiber and energy, and the techniques of bringing it all together, to get just the kinds of paper we want.