Mercury is the closet planet to the sun at a distance of 36 million miles. the closest star to the sun is proxima centauri, which is 4.24 light years away. a light year is 5.9 ­× 1012 miles.

So, if you are trying to convert lightyears to miles, you'll do so with this formula:
mi= (ly)* 5.9e^12, or 59,000,000,000,000. so it would be:
miles=(4.24)(5.9e^12)= roughly 2.5e^13   so Proxima Centauri is   250,000,000,000,000 miles from the Sun
To convert miles to lightyears, you'll just switch them around:
lightyears= (3.6e^6) / (5.9e^12)
Mercury is roughly 0.000006123880 lightyears from the Sun

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