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

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.