An armillary sphere (variations are known as spherical astrolabe, armilla, or armil) is a model of objects in the sky (in the celestial sphere), consisting of a spherical framework of rings, centred on Earth, that represent lines of celestial longitude and latitude and other astronomically important features such as the ecliptic. As such, it differs from a celestial globe, which is a smooth sphere whose principal purpose is to map the constellations. Beijing's Ancient Observatory was built in 1442 during the Ming Dynasty and is one of the world's oldest. It served the Ming and Qing astronomers in their star-gazing reports for the Emperor. As he was considered the Son of Heaven, the movements of the heavenly bodies were an important affair. Another function was to assist sea navigation, with Muslim scholars recruited for their expertise in the area.