How is one second time defined in SI system?

The second is defined in terms of a cesium-atom laser and its vibrations.

Explanation:

However, real time is defined in terms of the mean solar day, and it is thus necessary to add leap seconds to the atomic clocks to keep them in line with the astronomical clocks.

Hope this helped :)

answer: the correct answer is 1.

explanation: there are various process in which a radioactive nuclei decays:

1) alpha decay: in this process, a heavier nuclei decays into lighter nuclei by releasing alpha particle. the mass number is reduced by 4 units.

2) electron capture: in this decay process, a parent nuclei absorbs an electron and gets converted into a neutron. simply, a proton and an electron combines together to form a neutron. mass number does not change in this process.

3) gamma ray emission: it is a decay process in which an unstable nuclei gives excess energy by a spontaneous electromagnetic process. this decay releases . this process does not change the mass number.

4) positron emission: it is a type of decay process, in which a proton gets converted to neutron and an electron neutrino. this is also known as -decay. in this the mass number remains same.

from the above information, we see that only alpha-decay is producing a change in the mass number of the nucleus.

kepler used the data from tycho blahe (his mentor) to construct the laws of planetary motion.