Question
Define mean solar day. Explain the method for measurement of time.
✓
Answer
- A mean solar day is the average time interval from one noon to the next noon.
Method for measurement of time: - The unit of time, the second, was considered to be \frac{1}{86400} of the mean solar day, where a mean solar day = 24 hours
= 24 × 60 × 60
= 86400 s - However, this definition proved to be unsatisfactory to define the unit of time precisely because solar day varies gradually due to gradual slowing down of the Earth’s rotation. Hence, the definition of second was replaced by one based on atomic standard of time.
- Atomic standard of time is now used for the measurement of time. In atomic standard of time, periodic vibrations of caesium atom is used.
- One second is time required for 9,192.631,770 vibrations of the radiation corresponding to transition between two hyperfine energy states of caesium-133 (Cs- 133) atom.
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