What do “ asr al-awwal” and “ asr al-thani” mean?
“ Asr al-awwal” means the first time of ‘asr prayer and “asr al-thani” means the second time of ‘asr prayer. The time for ‘asr prayer starts when the time of dhuhr prayer ends. As the end time of dhuhr prayer is controversial among fuqaha due to difference in the evidences they use, the beginning of ‘asr prayer’s time is controversial as well. According to this, Imam Abu Yusuf, Imam Muhammad and imams of other three schools say that the time of dhuhr prayer ends and the time for ‘asr starts when the shadow of anything gets as long as itself except the shadow formed during the sun’s movement from the meridian to the west ( fay al-zawal) (Marghinani, al-Hidayah, I, 255-256; Shirbini, Mughni al-Muhtaj, I, 190; Dasuqi, Hashiyah, I, 177; Ibn Qudamah, al-Mughni, II, 12-14). This time is called “ asr al-awwal” (the first time of asr prayer).
According to Imam Abu Hanifa, the time of dhuhr prayer ends when the shadow of something gets two times longer than itself, except “ fay al-zawal”. This time is called “asr al-thani” (the second time of ‘asr prayer) (Kasani, Bada’i, I, 122; Marghinani, al-Hidayah, I, 255-256; Zaylai, Tabyin, I, 80).
Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS