Question

Which of the following is not a unit of time?
A. Leap year
B. Lunar month
C. Solar day
D. Parallactic second

Answer 1

Leap means to jump, lunar sounds have something to do with the moon, solar implies something about the sun and the parallactic second is employed in measuring large distances.

Complete step by step answer:

Option A: Leap year.
A leap year is a year that has an additional day so that the calendar year is in synchronous with the astronomical year or seasonal year. Each leap year has 366 days, instead of the usual 365 days, with February 29 as the extra day. It occurs once in every four years. It represents time and therefore, has a unit of time.

Option B: Lunar month.
A lunar month is the time between two successive new moons or full moons. The mean period of a lunar month is approximately 29.5 days. Its true period varies throughout the lunar year. It represents time and therefore, has a unit of time.

Option C: Solar day.
A solar day is the time taken for the Earth to rotate once around its axis relative to the Sun. It is the time between two successive noons. It represents time and therefore, has a unit of time.

Option D: Parallactic second.
One parallactic second is the distance at which the average radius of the Earth’s orbit subtends an angle of one arc second. The average radius of the Earth’s orbit is the average distance of the Sun from the Earth and is equal to one astronomical unit (1 AU).
The figure given below provides a better understanding

The parallactic second represents distance or length and therefore, does not have a unit of time.
Hence, the correct answer is option (D).

Note: Parallactic second may sound like a unit of time but it is not. When discussing astronomical units, the name of a unit does not always represent the expected physical quantity. Light year (i.e., the distance covered by light in a year) is also an example of such a confusing astronomical unit.