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What Is a Light Year?
© Copyright 1997, Jim Loy
Anyone reading this probably knows that a light-year is a unit of distance.
But much of the general public seems to think that it is a very long period of
time. In fact it is the distance that light travels in a year (about
6,000,000,000,000 miles). It is a particularly handy unit of measurement for
astronomers, because it also tells us how long ago the light that we see left
the particular object. If we look at the star Alpha Centauri, we see it as it
was a little over four years ago, for it took the light that long to get here.
The events (expanding and contracting) that we see on a variable star which is
1000 light-years away, happened 1000 years ago. And we see the Andromeda galaxy
as it was 2.2 million years ago. And we see distant galaxies that are billions
of light-years from us.
For the closest stars, the parsec is more often used. This too is a unit of
distance. When I first saw the movie Star Wars, I heard a groan from a
couple of members of the audience when Han Solo said that the Millennium Falcon
(his space ship) was the ship that made the Kessel run in 12 (as far as I
remember) parsecs. A parsec is the distance at which a star would have a
parallax of one second of arc. It is about 3.26 light-years (about
19,200,000,000,000 miles). The handy thing about a parsec is that when you know
the distance to a star in parsecs, you can estimate its parallax (1/distance),
or vice versa. A star's parallax, incidentally, is the amount that it seems to
move (against the background of distant stars) because of the earth's motion
around the sun. See the addendum, below. Only the nearest stars have measurable
parallaxes.
Addendum:
Here is a diagram, showing stellar parallax. We see
here that, as the Earth moves around the sun, a nearby star appears to move
against the background of distant stars. The closer this star is to us, the
greater the apparent movement. The angle of this movement is the parallax. This
angle is very tiny (the diagram exaggerates this angle, shortening the
distance), measured in seconds of arc. A star's other motions complicate this
slightly. Astronomers then subtract out these motions, which are real motions
through space, in order to find a parallax. As I say above, a parsec is the
distance at which a star has a parallax of one second.
Eric Idle, in The Road to Mars, mistakenly says that we measure time
in parsecs.
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