How can you find the parallax angle of a star?

1 Answer
Jan 9, 2017

See the explanation.


With respect to a star, the Earth E turns around its star Sun along

half of its nearly-circular orbit, of radius 1 AU, in one half year..

The diameter ( of length 2 AU ) of the Earth's orbit joining these

two positions P and Q subtends the so-called

parallax #angle PSQ = alpha#, at the star S.

Easily, the distance of the star ES, in AU unit, is given by

#1 AU = EP = ES tan (alpha/2)#.

As# (alpha/2)# radian is quite small for a distant star S,

#1 AU = ES (alpha/2)#, nearly.

In the case of the Sun, #alpha # is not small., and so, tan(alpha/2)

should be retained as such, and the distance reasd

ES = cot(alpha.2) AU = 1 AU.

So, #alpha/2=pi/4#, giving #alpha = pi/2#, ignoring small eccentricity

of the Earth's orbit.

Note that all these results are for half-year spacing PQ = 2AU,.