Who is "Rhadir of Seville"?

Within all probability, the most likely explanation seems to be that this is a mixup. The correct name should be: "Abū Muḥammad Jābir ibn Aflaḥ", a Muslim astronomer also known as latinised Geber.

Explanation: My searches for the exact name and variations came up all empty.

The text in question in English translation, names corrected, with added links to Wikipedia entries:

For years he researched with poorly capable instruments for such research. He did methodical empirical observation, and rigorous analysis of the classical theses that supported and opposed geocentrism — Pythagoras, Aristotle, Plato, Ptolemy, Eudoxus of Cnidus, Apollonius of Perga, and Aristarchus of Samos, Philolaus, Heraclides Ponticus, Rhadir of Seville, Nicolaus of Cusa, Peuerbach, Müller, respectively.

Nevertheless, Peuerbach as Feuerbach, Muller instead of Müller (German: Johann(es) Müller, from Königsberg, this then latinised to 'Regiomontanus') are indeed indicators of a certain sloppiness, as is Nicholas of Cusa (Latin alternative: Nicolaus Cusanus, German: Nikolaus von Kues).

So, while a few names of astronomers connected to Seville are relevant, the top few of Arabic heritage would be quite unrelated in sound patterns to 'Rhadir', with one probable exception.

'Radhir' appears to be more of an Indian name, rather than Arabic (or Spanish). For the desired timeframe, an Indian name in Seville seems not very likely. This again points towards some conflation. 'Rhadir' as a given name appears to be rare in Spain?. Search engines suggest 'Randhir' as an alternative, again pointing towards India for that. The rarity of results seem to indicate an extended typo.

Apparently the Arabic term in Latin transcription 'rhadir' exists, but means something like puddle?

That 'Rhadir' may be a typo for either Isidore of Seville or John of Seville seems out of the picture.

Well known Arab or Muslim astronomers connected to Seville:

• Abū Ishāq Ibrāhīm al-Zarqālī
• Ibn Rushd (Ibn Rushd (Arabic: ابن رشد‎; full name in Arabic: أبو الوليد محمد ابن احمد ابن رشد‎, romanized: Abū l-Walīd Muḥammad Ibn ʾAḥmad Ibn Rušd; 14 April 1126 – 11 December 1198), often Latinized as Averroes (English pronunciation: /əˈvɛroʊiːz/))
• Umawī: Abū Alī al-Ḥasan ibn Alī ibn Khalaf al-Umawī
• Ibn al-Kammād: Abū Jafar Aḥmad ibn Yūsuf ibn al-Kammād
• Ibn al-Hā’im: Abū Muḥammad Abd al-Ḥaqq al-Ghā qī al-Ishbīlī
• Jābir ibn Aflaḥ: Abū Muḥammad Jābir ibn Aflaḥ

The latter being known for:

Jābir ibn Aflaḥ was a mathematician and astronomer in 12th-century Andalusia, who wrote a treatise entitled Iṣlāḥ al-Majisṭi (Correction of the Almagest) in which, as the title suggests, the author made a long series of criticisms and corrections of Ptolemy’s main astronomical treatise.[…]

Little is known of Jābir’s life. It seems that he was from Seville, since he is referred to in several sources as al-Ishbīlī. One of these sources is Maimonides; in his Guide for the Perplexed, he claims to have met Jābir’s son. This reference suggests that Jābir was alive sometime between the end of the 11th century and the first half of the 12th century.

Jābir’s main work is a commentary on Ptolemy’s Almagest, a treatise that he had seen in two translations from the Greek. The Almagest is both the great synthesis and the culmination of mathematical astronomy of the ancient world, composed in Alexandria in the second century. It was translated into Arabic at least five times, and, from the late ninth century onward, constituted the basis of the mathematical astronomy carried out in the Islamic world.

In one of the preserved manuscripts (Berlin MS 5653), Jābir’s work appears under the title Iṣlāḥ al-Majisṭi (Correction of the Almagest); in fact, it is a reworking of Ptolemy’s work. Mathematical precision and proof seem to be Jābir’s maximum aspiration in his Iṣlāḥ.

In the Latin world, Jābir was considered a vigorous critic of Ptolemy’s astronomy. His treatise helped to spread trigonometry in Europe; in the 13th century, the trigonometric theorems were used by the astronomers who compiled the Libro del Cuadrante Sennero (Book of the sine quadrant) working under the patronage of King Alfonso X the Wise. In the 14th century, Richard of Wallingford used the theorems in his work on the Albion. Jābir is probably the source of much of Johann Müller’s (Regiomontanus’s) trigonometric work entitled De triangulis (On the triangles) although he is not mentioned. Finally, he may also be the source of the trigonometric section in Nicolaus Copernicus’s De Revolutionibus (On the revolutions [of the celestial spheres]).
_{— Thomas Hockey (Ed): "The Biographical Encyclopedia of Astronomers", 2007 Springer: New York, 2007.}

This is by far the closest sounding name for any astronomer connected to both Seville, the theories and names mentioned. (Example for a Spanish pronunciation of the latinised name)

The work was translated from the Arabic into both Hebrew and Latin, the latter by Gerard of Cremona, who Latinized his name as "Geber". (WP, and in Portuguese)

Other latinised versions of that name: (Arabic أبي محمد جابر بن أفلح الإشبيلي, DMG Abū Muḥammad Ǧābir b. Aflaḥ al-Išbīlī, and Gabir ben Aflah or Jabir ibn Aflah al-Ishbili.

Coincidently, when I now search just for "astronomer + Seville", Jabir is the second hit on the list. More about him in Salah Zaimeche: "The Scholars of Seville – Mathematics and Astronomy", Muslim Heritage, 15th August 2005.

And the direct connection to Copernicus:

It was in the twelfth century, in western Islam, that the first serious critics of Ptolemaic astronomy appeared. However, their objection were not based on new observations, but derived from philosophical considerations. Here I mention Jabir ibn Aflah of Sevilla, in Latin called Geber (not to be confused with the alchemist Geber) and after him al-Bitrfiji (Latin Alpetragius) who, doubtless unaware of the Eudoxian model, made a new attempt to represent planetary motion by the interaction of h*m*centric spheres. His work marked a regression rather than progress. Nevertheless it exerted an astonishing influence on scholastic and early Renaissance astronomers, which puts in evidence that their aim was also not to attain better agreement with observation but exclusively to devise models in better accord with the Aristotelian axioms.
_{— Willy Hartner: "Copernicus, the Man, the Work, and Its History", Proceedings of the American Philosophical Society, Vol. 117, No. 6, Symposium on Copernicus (Dec. 31, 1973), pp413–422.}

The astronomer “Geber” was cited as an authority in astronomy from the later middle ages until well into the sixteenth century. His name was associated with trigonometrical methods not found in the Almagest and with criticism of some of Ptolemy’s astronomy, particularly his placing Venus and Mercury below the Sun. Copernicus called him an “egregious calumniator of Ptolemy”
_{— R. P. Lorch: "The Astronomy of Jãbir ibn Aflah" Centaurus, vol. 19: no. 2: pp. p85–107, 1975.}

— Kevin Krisciunas & Beléen Bistúe: "Where Did Copernicus Obtain the Tools to Build His Heliocentric Model? Historical Considerations and a Guiding Translation of Valentin Rose’s 'Ptolemaeus und die Schule von Toledo' (1874)", arXiv preprint, arXiv:1712.05437, 2017 - arxiv.org.