### The Concept of Time through the Ancient and Modern Ages

Technically, no one discovered time, because time had always been there; so this exposition is for us to trace the development of the concept of time.

**Calendars**

For ancient cultures, time depended on the movements of heavenly bodies. There has been some evidence of the Moon being used to determine time as far as 6000 years ago. Since then, lunar calendars have been adopted by many cultures. It takes about 28 days for the moon to revolve around the earth, so roughly, there are 28 days in a month in a lunar calendar, and there are 12 x 28 = 342 days in a year. Because the moon has a shorter period of revolution than the month we now have and the movement of the moon was the sole determinant of the month, with no means or initiative to put additional months or days to some years, the arrangement of seasons (spring, summer, fall, and winter) became topsy-turvy. Some calendars included a thirteenth month to catch up (that makes it 364 days in a year).

The lunar calendar got so confounded over time such that the Roman emperor, Julius Caesar ordered a drastic change. He decreed that the basis of the year be not twelve or thirteen lunar months, but solely the revolution of the earth on the sun. The 365 days which took for the earth to the sun was divided by twelve months, so there will be remainders. The remainders were added to some months, so some months have 30 days; others have 31 days; there was one month, February (Februarius) that had 28 days. An extra day was added in February every four years, so that the calendar year will properly catch up with the tropical year (which is the cycle of the seasons).

However, this innovation by Julius Caesar had one error that is difficult to detect. It is that the Julian calendar year was 11 minutes longer than the tropical year. That made an error of 1 day per 128 days, but nothing was done about this for a long time. By 1582, there had been an error of extra 11 days in the Julian calendar, so Pope Gregory XIII proposed an ingenious measure: First, to cancel out the error, he declared that the next day after October 4, 1582, will be October 15, 1582. Next, to prevent future errors, he declared that years that end in two zeros are leap years only if they are divisible by 400. Thus, years like 1800, 1900, and 2100 are leap years under the Julian calendar, but not leap years under the Gregorian calendar. Years like 1600, 2000, and 2400 are leap years. Removing 3 days every 400 years amounted to a removal of 1 day every about 130 years, close to the quantity of error that the Julian system had (1 day every 128 years). With these, the Gregorian calendar was adopted by plenty of cultures and continues to be strikingly accurate up to now.

**Clocks**

Clocks also denoted the ancients' acquaintance with the concept of time as immediacy. Egyptians used sundials to calculate time based on the sun. Water clocks measured time even during night, but needed some manual maintenance to keep the water flowing. Hourglasses filled with sand could also measure time, but only for a limited moment.

Early devices held some inscrutable difficulties in accuracy. A sundial, the device most resembling a clock today, could be ahead or behind by as much as 15 minutes, depending on the Earth's position relative to the Sun. It was also ineffective when there is no sunlight. Clocks were also important in navigation, so clock accuracy is a renowned problem before. Today, clocks that meet modern standards of precision like the wall clock and the wristwatch are already powered by batteries.

Extremely precise clocks, like the atomic clock that relies upon frequencies of radiation released from a radioactive substance, have an error as small as one second every millions of years. The concept of time today has proceeded from heavenly bodies to extremely small entities today, like atoms; indeed, while the popular unit of time in antiquity had been the month, the SI unit of time today is the second, characterized by exactly 9 192 631 770 cycles or radiation released by a cesuim-133 atom.

### Websites For Learning All About Math Telling Time

- Daylight Saving Time
- The Galileo Project - Pendulum Clock
- A History of Time
- The Hourglass
- International Standard Date and Time Notation
- Latitude and Longitude
- Local Times Around the World
- Making a Horizontal Sundial
- Making a Sun Clock-
- National Geographic Kids Magazine: Water Clock Experiment
- On Time
- Time
- US Library of Congress - How does an hourglass measure time?
- What Time Is It!
- When Does Daylight Savings Time Occur