Measurement Time
by wang tong
1. why?
1.1. Easy for life
1.2. Easy to know time
1.3. Accuary
1.4. Meetings
2. Time Instruments
2.1. flow of sand in an hourglass
2.2. quartz watch
2.3. The 555 timer IC is an integrated circuit (chip) used in a variety of timer
2.4. Chip-scale atomic clocks
3. History
3.1. Egyptians were the first people who created a twenty-four hour day
3.1.1. Day was divided into ten hours
3.1.2. Night was divided into twelve hours
3.2. Same time.Chinese, Babylonians, Greeks and the Romans were using instruments to tell time
3.2.1. Chinese
3.2.2. Babylonians
3.2.3. Greeks
3.2.4. Romans
3.3. Egyptians and other ancient societies realized that the sun rose and set in different places in the summer and winter
3.3.1. put the post of the sundial in at a special angle
4. Methods
4.1. Since 1761
4.1.1. timekeeping has significantly changed
4.2. 1900
4.2.1. pendulum clocks had been finely tuned so as to only be off by 1/100 of a second each day
4.3. In the '20s
4.3.1. scientists discovered quartz crystals could keep even more accurate time than a pendulum and were only off about 1/500 of a second each year.
4.4. Half way through the 20th century
4.4.1. atomic clocks were built that would only be off by one second every 300 million years.