How is a parsec derived?

Answer 1

-- Hold your arm out straight, make a fist, then stick your thumb up straight,

like a gunsight.

-- Close one eye. Notice where your thumb appears, against the houses across

the street. Hold still and don't move.

-- Open the first eye, and close the other one. Notice again where your thumb

appears, against the houses across the street.

-- The two positions of your thumb, as seen with one eye at a time, are different.

If we measured the distance between your eyes, the length of your arm, and the

angle that your thumb seemed to move when you switched eyes, we could calculate

how far away those houses are from you.

-- The angle that your thumb seems to jump when you switch eyes is called "parallax".

We can use the same procedure to measure the distance to some stars, if they're relatively

close to us and in the right directions.

If we observe one of those stars twice ... six months apart ... it seems to shift its position

slightly, against the background of stars and galaxies that are really, really far away. That

happens because in six months, the Earth is way over on the other side of its orbit ... that's

186 million miles from the Earth's position 6 months earlier, and corresponds to the distance

between your two eyes. The star corresponds to your thumb.

If we know the distance across the Earth's orbit, and we measure the angle that the star

seems to shift in six months (parallax), then we can calculate the distance to the star.

(That corresponds to the length of your arm.)

A "parsec" is a distance. It's the distance to a star that has a parallax (appears to shift in

6 months) of 1 second of angle. The distance is about 3.26 light years. Anything closer to

us shifts more than 1 second of angle in 6 months, and anything farther away shifts less

than 1 second of angle in 6 months.

The nearest star outside our solar system is about 4.2 light years away. That means that

no star has a parallax as great as 1 second of angle. They're all less than that. And this

shows you why we can't use the same procedure to measure the distance to too many

stars . . . 1 second of angle is what you get when you cut 1 degree up into 3,600 equal

parts, and it's just so darn hard to accurately measure angles of 1 second or less.

The word "parsec" comes from "parallaxof onesecond".