Since 1967, the second has been defined as the duration of exactly 9,192,631,770 periods of the radiation corresponding to a hyperfine transition of cesium-133 in the ground state (BIPM 2006).

The second (s or sec) is the International System of Units (SI) unit of time measurement. One second is the time that elapses during 9,192,631,770 (or 9.192631770 x 10^{9} in decimal form) cycles of the radiation produced by the transition between two levels of the cesium-133 atom.

Since 1967, the second has been defined as exactly "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom".

A zeptosecond is a trillionth of a billionth of a second. That's a decimal point followed by 20 zeroes and a 1, and it looks like this: 0.000 000 000 000 000 000 001. The only unit of time shorter than a zeptosecond is a yoctosecond, and Planck time. A yoctosecond (ys) is a septillionth of a second.

The Persian scholar Al-Biruni first used the term "second" around 1000. He defined it—as well as the day, hour, and minute—as fractions according to the lunar cycle.

Following the tradition set by the Babylonians, these divisions were expressed using the sexagesimal system, a form of counting based on units of 60. Using this, the length of a second became a sixtieth of a sixtieth of an hour, leading to its definition as 1/3600th of an hour.

Seconds were once derived by dividing astronomical events into smaller parts, with the International System of Units (SI) at one time defining the second as a fraction of the mean solar day and later relating it to the tropical year.

Scientists have measured the shortest interval of time ever recorded, clocking how long it takes a particle of light to cross a single molecule of hydrogen. The ultra-quick journey took 247 zeptoseconds, according to a team of German researchers, with a zeptosecond representing a trillionth of a billionth of a second.

Their definition of a second is supposed to be exactly 1/86,400th of an average day — but it's based on an estimate of an average day in 1900, which was slightly too short. Days have generally been a bit longer since then, and a discrepancy has formed between solar time and official time.

Because it is the second division of an hour. The minute (the one everyone pronounces min-it) is the first minute (my-newt, or small) division of an hour into sixty parts. If you do a second division by sixty, you have… well, a second.

Who decided on these time divisions? THE DIVISION of the hour into 60 minutes and of the minute into 60 seconds comes from the Babylonians who used a sexagesimal (counting in 60s) system for mathematics and astronomy. They derived their number system from the Sumerians who were using it as early as 3500 BC.

We can never reach the speed of light. Or, more accurately, we can never reach the speed of light in a vacuum. That is, the ultimate cosmic speed limit, of 299,792,458 m/s is unattainable for massive particles, and simultaneously is the speed that all massless particles must travel at.

Scientists have recently observed for the first time that, on an epigenetic level, astronauts age more slowly during long-term simulated space travel than they would have if their feet had been planted on Planet Earth.

Light can travel at a speed of 300,000 km (186,000 miles) a second. That means that in one second, light travels the distance you would cover if you traveled around Earth 7.5 times! The moon is about one light second away.

The Egyptians broke the period from sunrise to sunset into twelve equal parts, giving us the forerunner of today's hours. As a result, the Egyptian hour was not a constant length of time, as is the case today; rather, as one-twelfth of the daylight period, it varied with length of the day, and hence with the seasons.

Historically, the word "minute" comes from the Latin pars minuta prima, meaning "first small part". This division of the hour can be further refined with a "second small part" (Latin: pars minuta secunda), and this is where the word "second" comes from.

At one-quintillionth of a second, an attosecond is unimaginably fast. In 53 attoseconds, light travels less than one-thousandth of the diameter of a human hair.

The measurements begin with the femtosecond (fs), which is 1,000 attoseconds. A picosecond (ps) is 1,000 femtoseconds. A nanosecond (ns) is 1,000 microseconds (µs). A microsecond is 1,000 nanoseconds, and a millisecond (ms or msec) is 1,000 microseconds.

Millisecond(One thousandth of a second) Microsecond(One millionth of a second) Nanosecond(One billionth of a second) Picosecond(One trillionth of a second)