== Astronomi ==
Aryabhata's system of astronomy was called the ''audAyaka system'', in which days are reckoned from ''uday'', dawn at ''lanka'' or " equator". Some of his later writings on astronomy, which apparently proposed a second model ( or ''ardha-rAtrikA'', midnight) are lost but can be partly reconstructed from the discussion in [[Brahmagupta]] 's ''khanDakhAdyaka''. In some texts, he seems to ascribe the apparent motions of the heavens to the Earth's rotation.
=== Mosi sistem suria ===
Aryabhata appears to have believed that the earth rotates about its axis. This is indicated in the statement, referring to ''Lanka '', which describes the movement of the stars as a relative motion caused by the rotation of the earth:
Like a man in a boat moving forward sees the stationary objects as moving backward, just so are the stationary stars seen by the people in Lanka ( or on the equator) as moving exactly towards the west." [''achalAni bhAni samapashchimagAni'' – golapAda.9]
But the next verse describes the motion of the stars and planets as real movements: "The cause of their rising and setting is due to the fact that the circle of the asterisms, together with the planets driven by the provector wind, constantly moves westwards at Lanka."
As mentioned above, ''Lanka'' ( lit. [[Sri Lanka]]) is here a reference point on the equator, which was the equivalent of the reference meridian for astronomical calculations.
described a [[ geocentric]] model of the solar system, in which the
Sun and Moon are each carried by [[epicycle]]s. They in turn revolve around
the Earth. In this model, which is also found in the ''Paitāmahasiddhānta'' (ca. CE 425), the motions of the planets are each governed by two epicycles, a smaller ''manda'' ( slow) and a larger ''śīghra'' ( fast).