Ancient astronomers were able to observe several astronomical phenomena before the advent of telescopes:
1) The rising and setting of the Sun in the east and west, and that the point of rising and setting varied throughout the year.
2) The phases of the Moon, from new to full and back to new over 29.5 days.
3) Lunar eclipses, where the Moon passes into Earth's shadow.
4) Solar eclipses, where the Moon passes between Earth and the Sun.