The constellation is determined by the date of the solar calendar. In 90 AD, the Gregorian calendar date on August 23rd of the lunar calendar is 10, 1 1, and the constellation of that day is Libra.
Lunar calendar: August 23rd, Year of the Horse.
Main branch: Geng Wubing, Ji Xuyou
Gregorian calendar:1990101October1Thursday Libra
Source: Hua Wang.