Question

In which of the following statements the word light year is used correctly?
$\left( {\text{A}} \right)$ Light travels with the speed of \[1\] light year in the Universe
$\left( {\text{B}} \right)$ Cetus dwarf galaxy is \[2.46\] million light year away from us
$\left( {\text{C}} \right)$ The next complete solar eclipse will be seen after \[32\] light year
$\left( {\text{D}} \right)$ The Earth takes \[\;365\] days to complete the distance of \[1\] light year around the Sun

Answer 1

Hint:
Light-year: It is a unit of length used to express astronomical distances and is equivalent to about \[9.46\] trillion kilometers \[\left( {9.46 \times {{10}^{12}}\;km} \right)\] or \[5.88\] trillion miles \[\left( {5.88 \times {{10}^{12}}\;mi} \right)\].
According to the International Astronomical Union (IAU), a light-year is defined as the distance that light travels in a vacuum in one Julian year \[\left( {365.25{\text{ }}days} \right)\].
The light-year is used when expressing distances to stars and other distances on a galactic scale.

Complete step-by-step solution:
Explaining option \[\left( {\text{A}} \right)\]: Light year is the measurement of distance not speed but according to the option it says the speed of light year in the Universe which is not true.

Explaining option \[\left( {\text{B}} \right)\]: Cetus Dwarf: It is a dwarf spheroidal galaxy. It is situated approximately \[2.46\] Million light-years from Earth. It is an isolated galaxy of the Local Group; it also contains the Milky Way. Mostly observable stars in the galaxy are red giants. So, this option is not true.

Explaining option\[\left( {\text{C}} \right)\]: Light year defines the distance but, in this option, it says about the time period of the next solar eclipse so this option is not true.

Explaining option \[\left( {\text{D}} \right)\]: It explains the light year definition correctly

So, this option D is correct.

Note:In a vacuum, light travels with the speed of \[\;670,616,629\] mph or \[1079,252,849\] km/h.
The speed of light is also referred to as the "cosmic speed limit".
Einstein first demonstrated it theoretically: that light is the fastest thing in the universe, travelling in the vacuum of space at a whopping \[300,000\] km per second (or \[186,000\] miles per second). Nothing can travel faster than that.