His best known work, Kitāb fī Jawāmiʿ ʿIlm al-Nujūmi (whose name translates to Elements of astronomy on the celestial motions), was an extensive summary of Ptolemy's Almagest containing revised and more accurate experimental data.

In addition to making substantial contributions to astronomy, al-Farghani also worked as an engineer, supervising construction projects on rivers in Cairo, Egypt.

[3][4] He was involved in the calculation of the diameter of the Earth by the measurement of the meridian arc length, together with a team of scientists under the patronage of the ʿAbbāsid caliph al-Ma'mūn in Baghdad.

[2][5] Al-Mutawakkil was enraged when he heard of the mistake, and so he sent Sanad ibn ‘Ali to assess the culpability of the brothers Muhammad and Ahmad who contracted al-Fraghani to build it.

[2] The most influential work produced by al-Farghani was his textbook Kitāb fī Jawāmiʿ ʿIlm al-Nujūm (كتاب في جوامع علم النجوم A Compendium of the Science of the Stars) or Elements of astronomy on the celestial motions, written sometime between about 833 and 857.

[6][13] The work Kitāb al-Fihrist by Ibn al-Nadim suggests that al-Farghani was also responsible for writing a book about the use and function of sundials, though no copies exist in the present day.

This error caused him to underestimate the Earth's circumference, leading him to sail to North America while he believed that he was taking a shortcut to Asia.