1. How the light years is calculated between earth and any other planet?

2.

3. Are you asking how we measure the distance to other celestial bodies? If so, it varies depending on what and how far away it is.

Check out this:
Cosmic distance ladder - Wikipedia, the free encyclopedia

4. Originally Posted by MOHANTHILAGARAJ
How the light years is calculated between earth and any other planet?
Well, let's see, light travels at 186,000 miles (300,000 km) per sec. X 60 seconds in a minute, X 60 minutes in an hour, X 24 hours in a day, X 365.25 days in a year = 1 light year.

Maybe Janus understood what you were asking better than I.

5. Thanks Janus! Yes, this is what I was looking for. BTW, I have another question: From one of the method of calculation mentioned in the article, if it is found that the distance of another celestial body is 100 light years from earth, then whether it would actually take 100 years for a light to reach that body? or more than that? Light years reference is "speed of light in vacuum" but the medium now is not vacuum.

6. Originally Posted by MOHANTHILAGARAJ
Thanks Janus! Yes, this is what I was looking for. BTW, I have another question: From one of the method of calculation mentioned in the article, if it is found that the distance of another celestial body is 100 light years from earth, then whether it would actually take 100 years for a light to reach that body? or more than that? Light years reference is "speed of light in vacuum" but the medium now is not vacuum.
From our perspective, it would, at least to within the error bars of our calculation (if you ignore expansion I presume). Interstellar space is not a perfect vacuum, but the particles that are present are so thinly spread, that they add virtually nothing to the travel time of light.

7. Originally Posted by KALSTER
Originally Posted by MOHANTHILAGARAJ
Thanks Janus! Yes, this is what I was looking for. BTW, I have another question: From one of the method of calculation mentioned in the article, if it is found that the distance of another celestial body is 100 light years from earth, then whether it would actually take 100 years for a light to reach that body? or more than that? Light years reference is "speed of light in vacuum" but the medium now is not vacuum.
From our perspective, it would, at least to within the error bars of our calculation (if you ignore expansion I presume). Interstellar space is not a perfect vacuum, but the particles that are present are so thinly spread, that they add virtually nothing to the travel time of light.
And to answer the question of how do we know this: Different wavelengths of light are slowed differently when passing through a medium. If there were any significant slowing of light due to this as it traveled from us to a distant star we would detect it as a time shift in the frequencies we see from it. For example we would see some colors brighten before others in distant supernovae.

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