Question

Which of the following is not a unit of time?
A. Mean solar day
B. Lunar month
C.Calendar year
D.Light year

Answer 1

Hint: Using the definitions we need to check which of the given options correctly represent time. So as we know that lunar month is the time interval between two successive new moons or full moons. And calendar year means the start and end of the month will be January and december. By using these points we can easily know the time unit.

Complete step by step answer: Out of the given four options we need to check which are the units of time.
So, we will define each of the options and check whether it is a unit of time or not.
Mean Solar Day is the average time taken by the earth to complete one rotation about its axis or it is the time interval between two successive transitions of the mean Sun across the meridian. It is approximately equal to $24$hours. Thus, it is clear that the mean solar day is a unit of time. Hence option A is a correct option.
Lunar month is the time interval between two successive new moons or full moons and hence is a unit of time.
Thus, option B is a correct option.
Calendar year is nothing but the time interval of $365$days in case of a non-leap year and $366$days in case of a leap year. So, basically it is the time between ${1^{st}}$January to ${31^{st}}$December of each year. Now whether we consider this time interval in terms of months, weeks, days, hours, seconds or milliseconds it always represents time. Hence, we can obviously say that the calendar year is a unit of time. Thus, option C is a correct option for this question.
Though from the name it may appear that light year is a unit of time, but actually it is not the case. According to the definition given by International Astronomical Union (IAU), a light year is the product of a Julian year ($365.25$days) and speed of light in vacuum ($299792458m{s^{ - 1}}$). As the product of time and velocity or speed gives distance, hence it can be concluded that light year is the unit of distance and not time. Thus, option D is not a correct answer for the given question.

Hence, option A, B and C are correct.

Note: Light year is mostly used to measure the distance of the astronomical bodies like stars and planets. Light-year measurement is also useful to decode how long ago in the past we are seeing. Allow me to explain. The distance, in light years, is the literal time it takes for the light from a star to reach the Earth in one year.