Why is the sky coordinate right ascension (RA) referenced to 1950.0 AD in most star atlases?
A) Celestial coordinates were first conceived by astronomers who worked in the middle of the 20th century and they decided the year 1950 was convenient
B) 1950 was the year that lateral stellar motion was discovered
C) The phenomenon of precession shifts the zero of RA over time
D) Telescopes were first constructed with hour circles and RA dials in the year 1950
E) 1950 was the year that telescopes improved to the point that astronomers could measure small shifts in the celestial sphere
Answer: B