The Andromeda Galaxy, also known as M31, is the nearest neighbour of our Milky Way Galaxy. But how far is our ‘nearest neighbour’ from Earth.
It’s a distance of 2.5 million light years. One light year is the distance travelled by light in a vacuum in one year. That means in one light year, light travels a distance of about 9,460,730,472,580.8 km (or 9.461 * 10^12 km).
In one million light years, light travels 9.461 * 10^18 km (here ‘^’ means to the power, hence 10 to the power 18 ).
So in 2.5 million light years, light travels a distance of 2.365 * 10^19 km.
Now if you send an email to a friend in Andromeda Galaxy, that travels with the speed of light, then it will need 2.5 million years to reach there. And your friend’s reply will take another 2.5 million years to reach you. So it’s a business of about 5 million years. Not baaad .
Although, M31 is so far, but it can be visible in naked eyes as a faint mark. That means the image we see is the ‘past’ of M31. Because light needed 2.5 million years to reach Earth from the Andromeda Galaxy. How it exactly looks today, we will know it after 2.5 million years.
Image: Great Andromeda Nebula. Published in 1899. Author: Isaac Roberts.
Source:A Selection of Photographs of Stars, Star-clusters and Nebulae, Volume II, The Universal Press, London, 1899.
So looking up to the Heaven we see not only our present day ‘Moon’ and ‘Sun’, but at the same time we see the ‘past’ of another celestial body (i.e. we are looking at the past from the present). That’s amazing.
Sources: International Astronomical Union (IAU), Wikipedia.
P.S. You can use Wolframalphato calculate these numbers.
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