The light year is a convenient unit for astronomy because it tells us not only a distance but also how far back in time one is looking.

Now that we have a good estimate for the age of the universe, let's say 14 billion years, a distance expressed in light years also tells us how old some objects like distant galaxies are: just subtract from 14 billion. For example, a galaxy 9 billion light years away cannot have stars older than 5 billion years, so no sun-like star in it can have left the main sequence portion of its lifespan yet.

Nevertheless, despite this convenience, astrononers seem to like to use parsecs and megaparsecs when talking about distances. But parsecs are less convenient when we also want the age of something, a problem which we now attempt to alleviate:

Much like how a year can be turned into a unit of distance by multiplying by the speed of light, we can turn a distance into a unit of time by dividing by the speed of light.

parsec/lightspeed = parsec/light = parsec/c = pc/c = 1.0292713e+08 second = 3.26 year

A megaparsec/c or Mpc/c is a million times more.

The age of the universe is 4230 Mpc/c. The age of the earth is 4.5 Gyr = 1400 Mpc/c.

Express your age in parsecs per c. 10 pc/c is a milestone.

Express the astronomical Julian Day in parsecs per lightspeed.

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