I have learned that it takes 23 hours 56 minutes 4 seconds for the earth to rotate on its axis. Why isn't it exactly 24 hours? What makes one second one second? I would like to know what the length of a unit of time is based on.
2006-12-05 13:40:07 · 5 answers · asked by Anonymous in Science & Mathematics ➔ Astronomy & Space
The average length of the day (sunrise to sunset) has been defined as 12 hours a long time ago. Why 12? Because it is a number easy to divide into 2,3,4 and 6.
The Romans divided the night also into 12 hours (to determine the length of night watches by soldiers). Thus the whole day (noon to noon) turns out to be 24 hours (a convenient number, easily divided into 2,3,4,6, 8,12...).
60 was a magic number (divisible by 2,3,4,5,6,10,12...). Ancient astronomers used a sexagesimal system where fractions were divided into 60's up to about 14th century (when some started to use the decimal system).
Scientists writing in Latin called the first divisions: pars minuta prima (the first small parts) from which we retained the word "minute". They called the second division; pars minuta secunda (the second small part) from which we retained the word second. Minuta had the same sense as our adjective "minute", as in "my piece of cake is minute; I will starve".
So, the average length of the Solar day, divided by 24 is an hour. One hour divided by 60 is a minute. One minute divided by 60 is a second. The names were used for any division of units. Today, the words still exist only for time and angles.
Recently, the second was redefined (as others have explained) since the Earth rotation is less regular than modern atomic clocks.
24 hours is the time it takes for one rotation in relation to the Sun.
As the Earth orbits the Sun once per year (365 days), it will turn on its axis 366 times for us to observe 365 days. Therefore, the difference between a day and a rotation of the Earth (in relation to the rest of the universe) is 1/366 of a day.
(1 day / 366)*(24 hours / day)*(3600 seconds / hour) = 236 seconds (3 min. 56 seconds). Therefore, one rotation (in realtion to the universe) takes 23h 56m 4s. During that time, the Earth has moved on its orbit (a little less than 1 degree since there are 360 degrees to cover in 365 days); thus the Sun is 1 degree shy of where it was the day before. It takes 4 minutes for the Earth to rotate 1 degree.
PS: When my piece of cake is minute, I have seconds.
2006-12-05 14:55:30 · answer #1 · answered by Raymond 7 · 0⤊ 0⤋
Hi. A star transit will measure the Earth's sidereal rotation. We use the sun transit which is less accurate to determine 24 hours and divide by 3600 to get seconds. 24 has the advantage of allowing 15 degrees of rotation for one hour. (24 times 15 = 360)
2006-12-05 13:44:16 · answer #2 · answered by Cirric 7 · 0⤊ 0⤋
A second is 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.
It is all right here:
http://en.wikipedia.org/wiki/Second
After atomic clocks came around, they stopped using the earth's rotation as the measurement.
2006-12-05 13:42:21 · answer #3 · answered by Anonymous · 0⤊ 0⤋
IM no rocket scientist but I would think its measured by the Sun its the only still object in our solar system, all planets rotate around it and turn at 1000 mph or something like that. not sure how far off i am on the speed but thats my understanding of it
2006-12-05 13:47:16 · answer #4 · answered by albert 4 · 0⤊ 0⤋
precise angles and extremely complicated formula.
2006-12-05 23:56:20 · answer #5 · answered by tim p 2 · 0⤊ 0⤋