How do astronomers determine whether to use astronomical units, light years or parsecs?

Answer 1

The smallest AU used for in-between distances of orbiters in a star system, like solar system. A much larger ly is used for stellar distances, Parsec is a little larger.

Mega parsec is used to calculate the distances of very distant galaxies; 1 AU = 149597870 km; 1 ly = 63242 AU, nearly; 1 parsec = 206265 AU, nearly. Parsec has an advantage over ly. It is related to angular spacing across for seemingly single but really globular groups of stars.

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Answer 2

Leah Abbate

Astronomers choose between astronomical units, light years, and parsecs based on the scale of the objects they are studying. They use astronomical units for measurements within our solar system, light years for distances to nearby stars and galaxies, and parsecs for more distant objects beyond the Milky Way.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.