1. [n]  ancient hawk-headed Egyptian sun god; a universal creator
2. [n]  (astronomy) the angular distance eastward along the celestial equator from the vernal equinox to the intersection of the hour circle that passes through the body; expressed in hours and minutes and second; used with declination to specify positions on the celestial sphere; "one hour of right ascension equals fifteen degrees"
3. [n]  an intensely radioactive metallic element that occurs in minute amounts in uranium ores

RA is a 2 letter word that starts with R.

1. \Ra\, n.
   A roe; a deer. [Obs.] --Chaucer.
   
   

2. \Ra-\
   A prefix, from the Latin re and ad combined, coming to us
   through the French and Italian. See {Re-} and {Ad-}.