牛郎織女距離多少光年？有人用幾何學算出來是14.81光年，我沒驗證。
https://ca.answers.yahoo.com/question/index?qid=20091116001038AAFEAbB
Are the stars Vega and Altair 16 light years away from EACH other?        

             Not quite. The distance between Vega and Altair is 14.81 light years.

Altair
r = 16.77 light years
α = 19h 50m 47.0s
δ = +8° 52' 6.0"

Vega
r = 25.3
α = 18h 36m 56.3s
δ = +38° 47' 01.3"

x₁ = r₁ cos α₁ cos δ₁
y₁ = r₁ sin α₁ cos δ₁
z₁ = r₁ sin δ₁

x₂ = r₂ cos α₂ cos δ₂
y₂ = r₂ sin α₂ cos δ₂
z₂ = r₂ sin δ₂

Δx = x₂ − x₁
Δy = y₂ − y₁
Δz = z₂ − z₁

d = sqrt[ (Δx)² + (Δy)² + (Δz)² ]

α = star's right ascension
δ = star's declination
r = star's distance from Earth
d = the distance between star #1 and star #2                                                         Dump the liberals into Jupiter                         · 6 years ago