distance to nearby stars

MathematicalPhysics

Just a little question, using the formula d=1/(theta), d is the distance in pc and theta is the angle in arsecs. Is theta simply the parallax? or if not, can it be calculated from right ascention and declination?

Thanks, Matt.

russ_watters

Its parallax.

selfAdjoint

You have to observe the star from two ends of the earth's orbit, in January and in June (aphelion and perihelion, the two ends of the semimajor axis of the orbital ellipse). Then you compute the angular distance between the two observations; that's your theta. You could do the computation by recording your two observations as right ascension and declination and then doing spherical trig to calculate the separation.