The astrological sign Leo is inevitably represented by the eponymous lion in medieval Books of Hours; the only variation is in the animal’s posture. He may be seen in profile, walking to the right or to the left, or, more rarely, he may be seated, evoking heraldic representations (Teresa Pérez-Higuera, Medieval Calendars, 1998). Here in the Belles Heures he is described by Timothy B. Husband as passant guardant (The Art of Illumination: The Limbourg Brothers and the Belles Heures of Jean de France, Duc de Berry, 2008).
The early medieval encyclopedist Saint Isidore of Seville, who often provides a mythological explanation for the images used to designate the signs of the zodiac, explained that the Greek hero Hercules killed an enormous lion, and that this beast was included among the twelve signs because of its great valor. When the sun reaches this sign, it releases its tremendous warmth on the world and causes the summer winds to blow. The thirteenth-century Breviari d’amor, an encyclopedic Occitan grammar composed by the Franciscan friar and troubadour Matfre Ermengau, explains that this sign is named for the lion because that beast is stronger and more vigorous than all the other animals and that the sun gives us greater strength and vigor when it enters Leo (Pérez-Higuera, Medieval Calendars).