From Longman Dictionary of Contemporary Englishzenithzen‧ith /ˈzenəθ $ ˈziː-/ noun [countable usually singular] 1 SUCCESSFULthe most successful point in the development of something SYN peak OPP nadirreach its zenith/be at its zenith The Roman Empire reached its zenith around the year 100.2 technicalHA the highest point that is reached by the Sun or the Moon in the skyRegisterZenith sounds rather literary. In everyday English, people usually say peak:Her career was at its peak.
Examples from the Corpus
zenith• When it had almost reached its zenith, the second rod also moved, so that the motion of the wheel was continued.• Wilson's career reached its zenith in 1978.• It reached its zenith Dec. 13, when the Dow industrials closed at 5216. 47.• I sat watching as the sun reached its zenith and the muezzin began to call the people to prayers.• It is with the porcupines, however, that the spiny defensive system reaches its zenith.• Couture, that zenith of craft, detail and construction, comes closest to approximating art in the most traditional sense.• Even at the zenith of my passion for Carolyn, I gave no thought to leaving.• The Sun can't be past, or not past, the zenith on the Sun.Origin zenith (1300-1400) Old French cenith, from Arabic samt (ar-ra's) “path (above the head)” 