# Parallax of a binary system

shirin
Hi
It is stated that:
"for a visual binary system, the measured trigonometric parallax is 0.3749+/-0.0057 arcsec."
I don't know this angle is parallax of what? I can guess it like this:
0.3749+0.0057 arcsec is the parallax of the whole system when they are at most distance of each other, which looks at a_1 + a_1*e + a_2 + a_2*e and,
0.3749-0.0057 arcsec is the parallax of the whole system when they are at less distance of each other, which looks at a_1 - a_1*e + a_2 - a_2*e.
Is this correct?

Thanks

lpetrich
It's the parallax across the Earth's orbit, just like the parallaxes of isolated stars.

shirin
and why is it +/-something? Is this related to error?

Staff Emeritus