I. Detailed explanation
1, calculation process: (5-2)×5= 15 (seconds)
2. It is known that there are generally 12 numbers on the dial of clocks and watches, while * * has 12 intervals. When the second hand rotates for 60 seconds, it can be known that 60÷ 12=5 seconds, that is, each interval represents 5 seconds, and there are 3 intervals from the number 2 to the number 5, so 3 × 5 = 65438.
Second, the second unit and conversion
1, in milliseconds, microseconds, nanoseconds, picoseconds, femtoseconds, attosecond, milliseconds, milliseconds.
2. The second is the basic unit of time in the international system of units, with the symbol S, sometimes marked with English abbreviations as seconds. .
3. 1s= 100 μ sms (milliseconds) = 1 00000μ s (microseconds) = 1 000000ns (nanoseconds) = 1 000.
The origin of "second":
The origin of 1 and "second"
Before the Han dynasty, "time" refers to the season, and "time" is equivalent to the current season. There are four seasons in a year, so a year is also called "four seasons".
After the Han dynasty, "time" no longer represents the season, but is used to represent the unit of calculating time. At that time, people divided a day into 12 hours on average, and "one hour" was equal to two hours now. People are used to calling this "hour".
2,60 seconds
Decimal came from Babylon, and they used 60 as the unit of calculation. But the Babylonians didn't divide the time into 60 minutes, while the ancient Egyptians divided the day into days with 12 and the night into nights with 12. They also divided the four seasons in this way.
Ancient Greek astronomers, including Bruno Valli and Ptolemy, defined a quarter of the solar day as time. When subdivided by sexagesimal, the second is 86,400th of a solar day.
3. Calendar seconds
1956, the second is defined as the period of revolution of the earth in a specific era, because astronomers know that the rotation of the earth on the rotation axis is not stable enough to be used as the standard of time.
One third of the tropical year of 3 1.556925.9747 is one second from Gregorian calendar 1900 1.02. In 1960, this definition was adopted by the 1 1 International Conference on Weights and Measures.