Alfonso X, El Sabio, the Learned, the Astronomer, or the Wise (November 23, 1221 – April 4, 1284) was a king of Galicia, Castile and León (1252 - 1284). He is perhaps the most interesting, though far from the most capable, of the Spanish kings of the Middle Ages. His nickname "El Sabio" usually means "the Wise", but in his case, a translation "The Learned" is more accurate.

He was a writer, and he had considerable scientific fame, based mainly on his encouragement of astronomy and the Ptolemaic cosmogony as known to him through the Arabs. The Alphonsus crater on the Moon is named after him. After hearing an explanation about Ptolemy's theory of epicycles he is credited to have said: "If the Lord Almighty had consulted me before embarking on creation I should have recommended something simpler".

He established at Toledo a translation school that did a great work increasing the flow of knowledge into Christian Europe. Much of it was based on Ancient philosophy. His fame also connects itself with the preparation of the Alfonsine tables.

As a ruler he showed legislative capacity, and a very commendable wish to provide his kingdoms with a code of laws and a consistent judicial system. The Fuero Real was undoubtedly his work, and he began the code called the Siete Partidas, which, however, was only promulgated by his great-grandson. Alfonso was the first king who initiated the use of Castellano (Spanish) instead of Latin as the language of culture (culture referring to the language used in courts, churches, and in books and official documents). He lacked the singleness of purpose required by a ruler who would devote himself to organization, and also the combination of firmness with temper needed for dealing with his nobles. His descent from the Hohenstaufen through his mother, a daughter of the emperor Philip of Swabia, gave him claims to represent the Swabian line. Being elected by the German electors, after the death of Conrad IV in 1254, misled him into wild schemes which never took effect but caused immense expense. To obtain money he debased the coinage, and then endeavoured to prevent a rise in prices by an arbitrary tariff. The little trade of his dominions was ruined, and the burghers and peasants were deeply offended. His nobles, whom he tried to cow by sporadic acts of violence, rebelled against him.

Alfonso's eldest son, Ferdinand de la Cerda, died in 1275, leaving two infant sons. Alfonso's second son, Sancho, claimed to be the new heir, in preference to the children of Ferdinand de la Cerda, basing his claim on an old Castilian custom. Alfonso preferred to leave the throne to his grandsons, but Sancho had the support of the nobility. A bitter civil war broke out, but in 1282 Alfonso was forced to accept Sancho as his heir in preference to his young grandsons.

Son and nobles alike supported the Moors, when he tried to unite the nation in a crusade; and when he allied himself with the rulers of Morocco they denounced him as an enemy of the faith. A reaction in his favour was beginning in his later days, but he died defeated and deserted at Seville, leaving a will by which he endeavoured to exclude Sancho and a heritage of civil war.
In addition to his other achievements, Alfonso X commissioned or co-authored numerous works during his reign, including the Cantigas de Santa Maria (400+ songs mentioning the Virgin Mary, Galician-Portuguese), Cantigas d'escarnio e maldicer (also in Galician) and the Libro de los juegos, or "Book of Games".

In 1246, Alfonso X married Violante of Aragon, the daughter of King James I of Aragon and Yolande of Hungary. Because of her young age (Violante was only 10-years-old at the time of the marriage) she produced no children for several years and it was feared that she was barren. Alfonso almost had their marriage annulled, but they went on to have twelve children:
1. Fernando, died in infancy, and buried at Las Huelgas in Burgos.
2. Berengaria of Castile (October or November 1253-after 1284). She was betrothed to Louis, the son and heir of King Louis IX of France, but her fiance died prematurely in 1260. She entered the convent at Las Huelgas, where she was living in 1284.
3. Beatriz of Castile (November or December 1254-1280). She married William V of Montferrat.
4. Ferdinand de la Cerda (October 23, 1255-July 25, 1275). He married Blanche, the daughter of King Louis IX of France, by whom he had two children. Because he predeceased his father, his younger brother Sancho inherited the throne.
5. Leonor of Castile (1257-1275)
6. Sancho IV of Castile (May 13, 1258-1295)
7. Constanza of Castile (1258-August 22, 1280), a nun at Las Huelgas.
8. Pedro of Castile (June 1260-October 10, 1283)
9. Juan of Castile (March or April, 1262-June 25, 1319).
10. Isabella, died young.
11. Violante of Castile (1265-1296). She married Diego Lopez de Haro
12. Jaime of Castile (August 1266-August 9, 1284)

Alfonso X also had several illegitimate children. His illegitimate daughter, Beatriz, married King Alfonso III of Portugal. An illegitimate son, Martin, was Abbot of Valladolid.

Additional notes
A portrait of Alfonso X hangs in the chamber of the United States House of Representatives.