You have often heard that whenever the distance of a star or planet is measured from the earth, it is always done in light years. But have you ever wondered why the universe is always measured in light years instead of using kilometers?
Light is such a thing without which the whole space science is incomplete. Even life is not possible without light as it is the only form that gives energy to life. Even though life might have been possible once without light, but without it, we can never understand the universe. Everyone knows that light travels through space at just over 186,000 miles per second (300,000 km/sec). Now if you take your first step in space science, the very first word you'll hear is "light year".
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If we explain it in very simple language, then "the distance that light travels in space within one year of the earth, i.e., 365 days, is called a light year". Meaning, if any light emerges from the earth today, then after one year the distance it will cover in space will be called a light year. And there are about 9.5 trillion kilometers in a light year.
So what is this distance, after all? You can understand it in such a way that it takes 1.3 seconds for the light to travel from the earth to the moon. Whereas it takes 8 minutes and 20 seconds for the light to reach the earth from the sun. And it takes 4 hours for the light to reach Neptune from Earth, the last planet in the solar system.
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But for light, it is a very small distance, so now let's take a slightly longer distance. Take for example the closest star to the Sun, Proxima Centauri, which is about 40.17 trillion kilometers away from us, i.e. 4.25 light years. It is so far away that even if we were to reach this star at the speed of Voyager 1 (61,500 km/h), it would still take 70,000 years to reach there. Now if we compare the Milky Way galaxy, it is spread over a diameter of about 105,700 light years, and our entire universe (observable universe) is currently 93 billion light years across. Also, light-years allow us to measure the size of massive celestial structures such as galaxies and black holes.
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