Answer
The solar day, also the interval between 2 consecutive crossings a particular longitudinal line by sun, is used to define second.
1 second is equal to $\frac{1}{86400}$ of a solar day. But due to elliptical path taken by earth there is variation in length of a day.
Second defined by mean solar day which is calculated from the solar year is also not accurate due to variations in motion of earth.
These definitions are rather replaced by atomic standards. As per atomic standards,1 second is equivalent to 9192631770 oscillations of Cs-133 atom. Cs-133 atomic fountain clock is still being used to define second in which there is a drift of less than 1 second in 20 million years.
Work Step by Step
The solar day, also the interval between 2 consecutive crossings a particular longitudinal line by sun, is used to define second.
1 second is equal to $\frac{1}{86400}$ of a solar day. But due to elliptical path taken by earth there is variation in length of a day.
Second defined by mean solar day which is calculated from the solar year is also not accurate due to variations in motion of earth.
These definitions are rather replaced by atomic standards. As per atomic standards,1 second is equivalent to 9192631770 oscillations of Cs-133 atom. Cs-133 atomic fountain clock is still being used to define second in which there is a drift of less than 1 second in 20 million years.
