How far are we from the sun? How far are the planets away from each other? The distance between the sun and earth is about 149,600,000 Km. The distances involved are so very huge that astronomers do not measure them in miles or kilometers. If it is done so, then they have to keep a note of a 15-20 digit number which becomes very complex while trying to calculate astronomical distances. Hence the use of the term ‘light year’ came into being and is very commonly used in astronomy as a measurement of distance as the distance between objects in the outer space.
Have you wondered how much time the light takes from the sun to reach earth? The answer is 8 minutes. With such an enormous distance, light just takes about 8 minutes because light travels at a great speed of 299,792,458 meters/second. If light travels 299,792,458 meters per second, the distance it can travel in one year is called one light year. One light year is defined as 5,865,696,000,000 miles or 9,460,800,000,000 kilometers (10 trillion km approximately). Thus the huge astronomical distances can be easily measured in terms of light year. For instance, we could just say the stars nearest to our sun in the Milky Way are 4.3 light-years away instead of 4.06802721 × 1013 ( 10 raised to 13) kilometers. The whole of our galaxy is about 100,000 light years in diameter.
We know that stars emit light which in turn makes the star visible to our eyes. Suppose say the star we see today in the sky is some 1000 light years away from us. This means that light has travelled 1000 years to reach the earth. So this means that the star we actually see today is how the star looked 1000 years ago and not how it looks today.