Question

When calculating distance between the sun and earth, what are the advantages of using the parsec over the lightyear?

Answer 1

A light year is the distance it takes light one year to travel. It takes light from the Sun about 5.5 hours to reach Pluto when it is 40 AU a way. The nearest star to us is 4.2 light years away and hence light year is a better unit of distance for stars.

A parsec is 3.26 light years is equally good for measuring such distances. It comes from two words 'parallax' and 'second', the latter referring to measurement of angle and not time. For measuring this, astronomers position a star once (using sophisticated instruments), and then again 6 months later (against far more distant stars), when earth has moved on the opposite side of its orbit and calculate the apparent change in position.

So a distance of one parsec is one at which earth's orbit subtends an angle of one arc second and there is an inverse relation between angle and distance in parsecs. For example, distance of two parsecs is one at which earth's orbit subtends an angle of half of an arc second.

Hence while light year as unit of distance, is of the same order as parsec. the latter translates directly from measured parallax and hence is preferred among astronomers.