The distances of the universe are so great that ‘kilometre’ is an extremely small and inconvenient unit to express such large distances. The extremely large distances between the various heavenly bodies like the stars and planets can be expressed in terms of two units : (i) Light year, and (ii) Parsec.

One light year is the distance travelled by light in one year (which is equal to 9.46 x 10^{12 }kilometres). Let us see how we get this figure. Now, the speed of light is 300,000 kilometres per second (km/s). So, a light year is equivalent to 300,000 multiplied by the number of seconds in one year which is 365 x 24 x 60 x 60 (365 days, 24 hours, 60 minutes, 60 seconds). Thus,

1 Light year= 300,000 x 365×24 x 60×60 km

or 1 Light year = 9.46 x 10^{12} kilometres

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It is obvious that light year is a unit of distance and not of time. The nearest star to the earth (apart from the sun) is ‘alpha centuari’ which is about 4.3 light years away from the earth. Thus, the light coming from this star which we will see tonight would have left the surface of this star 4.3 years ago! Let us find out the actual distance of the alpha centuari star from the earth in kilometres.

Now, 1 Light year = 9.46 x 10^{12} km

So, 4.3 Light years = 9.46 x 10^{12} x 4.3 km

* = *40.67 x 10^{12} kilometres

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Astronomers also use another unit for measuring large distances which is called parsec. Parsec is an astronomical unit of distance which is equal to 3.26 light years.

That is, 1 parsec = 3.26 light years.