Have you ever heard astronomers use the term
"parsec" to describe distances in space?
More importantly, have you ever heard that
the Millennium Falcon can make the Kessel
Run in less than 12 parsecs?
What the heck does this word mean?
A parsec is an astronomical term for measuring
distance in space.
A single parsec is approximately 30.9 trillion
kilometers, or 3.26 light years.
In other words, it takes photons of light
over three years to travel a parsec's distance.
Parsecs are used for measuring really really
large distances.
Here in our Solar System, we usually use kilometers
or astronomical units.
In the distances between stars, astronomers
measure distances in parsecs, and for the
vast gulfs between galaxies it's kiloparsecs,
megaparsecs and gigaparsecs.
It seems like a strange number, though, 3.26
light years.
So where does the concept of a parsec come
from?
Here's the technical term:
"a parsec is the distance corresponding to
a parallax of one arcsecond"
Need more information than that?
Okay fine.
Consider how the Earth travels around the
Sun in its orbit.
At one point in the year, it's on one side
of the Sun, and then 6 months later, the Earth
has traveled to the opposite point in its
orbit.
From our perspective here on Earth, a nearby
star will appear to move back and forth a
tiny little bit against the background Universe.
You can get this same effect by holding up
your thumb, staring down your arm, and then
opening and closing your eyes.
See how your thumb is moving back and forth
compared to the background scenery?
This is known as parallax.
When a foreground object moves compared to
a more distant background.
By measuring the changing angles from the
observer to the star and the background Universe,
you can calculate the distance.
Imagine a right-angle triangle, the bottom
of the triangle is the distance from the Earth
to the Sun, also known as an astronomical
unit.
The side of the triangle is the distance from
the Sun to the star.
And then there's the angle required to make
the star appear to move 1 arc second in the
sky.
The sky is broken up into 360 degrees.
Each degree is broken up into 60 arc minutes,
and there are 60 arc seconds in an arc minute.
So one arc second is 1/3600th of the sky.
For example, the closest star in the sky,
Proxima Centauri, has a parallax measurement
of 0.77233 arcseconds -- that's how far it
shifts in the sky from when the Earth shifts
its position by 1 astronomical unit.
If you put this into the calculation, you
determine that Proxima Centauri is 1.295 parsecs
away, or 4.225 light years.
To give you another comparison, Saturn appears
to be about 14 arc seconds when it's at its
most distant point in the sky, so astronomers
are measuring really tiny distances.
So now I hope you have a better idea of what
a parsec is, and how astronomers use it to
measure distances in the Universe.
But what does it means to make the Kessel
run in under 12 parsecs?
Well, that's another episode.
