Arab astronomers made significant contributions to astronomy, but they did not determine that the Earth was a sphere. This understanding predates them by centuries. The ancient Greeks, particularly Pythagoras and Aristotle, were among the first to propose the Earth's spherical shape based on observations like the curved shadow the Earth casts on the Moon during a lunar eclipse and the changing constellations as one travels north or south.

However, Arab astronomers did make crucial advancements in astronomy, including:

* Refining astronomical calculations and instruments: They perfected methods for calculating the positions of stars and planets, leading to more accurate star catalogs and improved instruments like the astrolabe.

* Developing new theories: They made important contributions to the understanding of planetary motion and contributed to the development of the geocentric model of the universe.

* Preserving and translating ancient Greek texts: Arab scholars played a vital role in preserving and translating ancient Greek astronomical texts, making them available to later generations of scientists.

Therefore, while Arab astronomers did not determine the shape of the Earth, their contributions were crucial for the advancement of astronomy and laid the groundwork for future discoveries.