With the advent of the optical clocks, that are thousand times more accurate than the atomic clocks that were used to measure time since '67, the duration of a second is gonna be redefined soon.
I've put together what I've gleaned, about the subject, from the internet. And here, I go:-
* In 1835, the astronomer Simon Newcomb published his 'Tables of the Sun' based on the
observations of the sun's position during the years 1750 to 1892. The calculations were
reliable and in use till the '80s(The mean solar period was taken to be the middle of the
period viz. 1820).
* Before '56, the second was defined as the mean solar second or 1/86,400th of the time taken
by the earth to rotate on its own axis and see the sun again, each day. But the moon's gravity
and the tides slow down the earth's spin, whereby this is not a stable quantity or a constant.
* The astronomers redefined the second to be the 'ephemeric second' based on the speed of the
earth's orbit around the sun, in 1900, as predicted in Newcomb's tables. The earth's orbit around
sun does not slow as the earth's spin on its own axis(The ephemeric second is measured by
observing the moon's orbit around the earth, along with the picture of what stars the moon is
near).
* From '52 to '58, the astronomers measured the frequency of Cesium oscillations in terms of the
ephemeric second. Cesium is even more stable than the orbit of the earth around the sun, giving
results between 9,192,631,761 and 9,192,631,780. The average of 9,192,631,770 oscillations is
the duration of a second as of now!
* Why was Cesium chosen?
Based on the principle of an atomic clock, the frequency of the splitting of energy levels caused
by the magnetic field of the nucleus and for this to work, the following were required:-
(i) an atom that can easily be vaporised.
(ii) an atom with a magnetic field(Must contain odd number of protons/neutrons).
(iii) an atom with just one stable isotope(So you don't have to purify it) and
(iv) a high frequency for the transition to enable more accurate measurement in shorter
time).
And therefore, Cesium Cs 133 became the standard!
Tailpiece.
Indira kunjamma had come yesterday afternoon, along with her friends, Jayanthi and Bina to collect the papers concerning the booking of the 'Aadhyaathmika Hall'(The prayer hall atop the outer ring of the sanctum sanctorum) for the 'Narayaneeyam recital' from 0600 to 1300 hrs, today. They're a total of 10 ladies, all from Thiruvananthapuram and returned by the Jana Shatabdi, that left Thrissur, at 1556 hrs today.
I've put together what I've gleaned, about the subject, from the internet. And here, I go:-
* In 1835, the astronomer Simon Newcomb published his 'Tables of the Sun' based on the
observations of the sun's position during the years 1750 to 1892. The calculations were
reliable and in use till the '80s(The mean solar period was taken to be the middle of the
period viz. 1820).
* Before '56, the second was defined as the mean solar second or 1/86,400th of the time taken
by the earth to rotate on its own axis and see the sun again, each day. But the moon's gravity
and the tides slow down the earth's spin, whereby this is not a stable quantity or a constant.
* The astronomers redefined the second to be the 'ephemeric second' based on the speed of the
earth's orbit around the sun, in 1900, as predicted in Newcomb's tables. The earth's orbit around
sun does not slow as the earth's spin on its own axis(The ephemeric second is measured by
observing the moon's orbit around the earth, along with the picture of what stars the moon is
near).
* From '52 to '58, the astronomers measured the frequency of Cesium oscillations in terms of the
ephemeric second. Cesium is even more stable than the orbit of the earth around the sun, giving
results between 9,192,631,761 and 9,192,631,780. The average of 9,192,631,770 oscillations is
the duration of a second as of now!
* Why was Cesium chosen?
Based on the principle of an atomic clock, the frequency of the splitting of energy levels caused
by the magnetic field of the nucleus and for this to work, the following were required:-
(i) an atom that can easily be vaporised.
(ii) an atom with a magnetic field(Must contain odd number of protons/neutrons).
(iii) an atom with just one stable isotope(So you don't have to purify it) and
(iv) a high frequency for the transition to enable more accurate measurement in shorter
time).
And therefore, Cesium Cs 133 became the standard!
Tailpiece.
Indira kunjamma had come yesterday afternoon, along with her friends, Jayanthi and Bina to collect the papers concerning the booking of the 'Aadhyaathmika Hall'(The prayer hall atop the outer ring of the sanctum sanctorum) for the 'Narayaneeyam recital' from 0600 to 1300 hrs, today. They're a total of 10 ladies, all from Thiruvananthapuram and returned by the Jana Shatabdi, that left Thrissur, at 1556 hrs today.
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