The earliest astronomical instruments in India are the śaṙku (gnomon) and the ghaṭikā (clepsydra). The former is mentioned in the Śulbasūtras, and the latter in the Vedāṅqajyotiṣa. Aryabhaṭa described a rotating model of the celestial sphere. After Aryabhaṭa, several instruments were described by Varāhamihira, Brahmagupta,Lalla, Śrīpati , and Bhāskara II. After Bhāskara II , some Sanskrit texts specialized on astronomical instruments were composed. The earliest text of this kind is the Yantra-rāja (AD 1370) written by Mahendra Sūri. It is also the first text on the astrolabe in Sanskrit. After Mahendra Sūri, Padmanābha, Cakradhara, Ganeśa-Daivajña etc. composed Sanskrit texts on instruments, but most of them remain unpublished.