km, miles; AUs; light seconds, light minutes, light days, etc; light years (ly), kilo light years (kly), etc; parsec (pc), kpc, Mpc, Gpc
absolute vs. apparent magnitude/brightness/luminosity; distanc module m-M
m-M = 5 lg(D/10 pc) = 5 lg(D/pc) - 5 = 5 lg(D/ly) - 7. D = 10 pc * 10^((m-M)/5)
d/D = 2 tan (Dim/2) ~ arc Dim D = d [cot(Dim/2)/2] ~ d / arc Dim
Distance determinations in the Solar System have been performed in the past by parallax measurements from different points on Earth (in former centuries, risky and expensive sea journeys were done for this purpose).
Nowadays, distances in the solar system are very acurately determined by radar methods, often involving spacecraft.
Ursa Major Moving Cluster:
about 60 stars at 78 ly
Scorpius-Centaurus cluster: about 100 stars at 560 ly; includes a number of B and A stars
(Pleiades estimated by Van Leeuwen at 126 pc, 410 ly)
Important if other distance values not available for certain object type (B stars, RR Lyrae, Cepheids)