While only seven planets were known by medieval Arab astronomers, an invisible eighth planet called al-tinnin or al-jawzahr (the dragon) was thought to cause solar and lunar eclipses. According to Hindu mythology from which this idea may have originated, the planet, which was described in Arab texts as a dragon monster, periodically devoured the sun or moon. Two medallions on this mortar depict the planet, shown here in the form of a seated figure flanked by dragon-headed snakes.