# Parallax and distance

1. ### fatima_a

24
A star exhibits a parallax of 0.250 arcseconds when measured from Earth.
a) What is its distance (pc)?

I know this uses the formula d=1/p, so i can solve this very easily

b) What would its parallax be if measured from Venus’s orbit (arcseconds)? (Look up
the one parameter you need to solve this using the relevant equation.)

I don't know what to do here at all though, I can't look up it's distance from Venus because its an arbitrary star...so what do i do? Can someone guide me to the right direction??

Thanks

2. ### Bandersnatch

1,258
What is the base of the triangle used to measured parallax of distant stars?

(by the way, you should post these kind of questions in the homework section)

3. ### HallsofIvy

40,310
Staff Emeritus
Do you know what parallax means? Imagine lines drawn from this "arbitrary star" to opposite ends of Venus' orbit (on the line through the sun perpendicular to the line to the star). That gives you an isosceles triangle with one angle (the one at the star) known and one side (the distance across Venus' orbit). You can use the "cosine law" to find the lengths of the two other sides (the distance from Venus to the star).

"I know this uses the formula d=1/p".

There's your problem- you know formulas but you don't know the reasoning behind the formulas so you don't know how to change them when the situation changes!

4. ### Simon Bridge

14,660
Just to be clear here - do you know how the equation d=1/p is derived?
i.e. Do you know the relationship between parallax measurements and triangulation?

HallofIvy beat me to it.