Question

What is the speed of light in parsecs per year? What is a parsec?

Asked by: Rohit

 Answer

A parsec is a distance of about 3.26 light years. Its name comes from 'PARalax SECond' because a star at that distance would appear to shift by one second (1/60 of 1/60 of one degree) against the background of stars from opposite sides of the Earth's orbit. The amount of that apparent shift is how the distance to nearby stars is determined.

One light year is the distance light travels in one year, so it would take light 3.26 years to travel one parsec. One parsec per 3.26 years is the same as .307 parsecs/year, the speed you asked about.

Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor


A parsec is the distance a star has to be from earth so that its parallax is one arcsecond. What in the world does this mean? Well, Parallax is what happens when you hold your thumb at arm's length from your face and look at it against the background of your room first with one eye open and the other eye closed and then the other eye open and the one eye closed. You will note different backgrounds to your thumb as a result of the slight difference in the relative position of your thumb and each eye.

Now, imagine a star. If you look at a star in June and then look at the same star in December, when the Earth is across its orbit you might see a different background of stars. The star will appear to shift relative to the 'fixed' background of stars. At what distance from the earth would this star have to be so that the shift was one second of 360 degrees of arch? That distance is a parsec. Do you see the 'sec' in parsec? Do you see the 'Par' from Parallax?

Answered by: Tom Young, B.S., Science Teacher, Whitehouse HS

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