| Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī | |
| Native Name: | خالد بن عبدالملك المرو الروذي |
| Native Name Lang: | ar |
| Birth Date: | fl. 9th century |
| Era: | Islamic Golden Age |
| Main Interests: | Astronomy |
Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī (Arabic: خالد بن عبدالملك المرو الروذي) was a 9th-century Baghdadi astronomer.
In 827, Marwarrūdhī, together with the astronomer ʿAlī ibn ʿĪsā al-Asṭurlābī and a party of surveyors, measured the length of a meridian arc of one degree of latitude. The party travelled to the Nineveh Plains in the valley of the Tigris, at 35 degrees north latitude. The measurement they obtained enabled the astronomers to obtain a value of for the circumference of the Earth, (or, according to other sources,). The two researchers measured in Arabian, and determined the geographical latitudes of the end points they used from the star altitudes in a celestial horizontal coordinate system. As it is thought that one Arabian represented, they found the length of 1° of meridian to be, which differs from the true value by .
Marwarrūdhī was chosen by the geometer Al-Abbās ibn Said al-Jawharī to organize a new observatory on Mount Qasioun. Despite encountering technical difficulties caused by the distortion of the astronomical instruments, in he spent a year obtaining of solar and lunar observations of the Sun and the Moon. He played a role in the project determine the length of the spring season by means of astronomical observations.
Marwarrūdhī was the first of three generations of astronomers.