The sun-god, who controls the affairs of the entire universe, especially in regard to heat, light, seasonal changes and so on, is considered an expansion of Nārāyaṇa. He represents the three Vedas — Ṛg, Yajur and Sāma — and therefore he is known as Trayīmaya, the form of Lord Nārāyaṇa. Sometimes the sun-god is also called Sūrya Nārāyaṇa. The sun-god has expanded himself in twelve divisions, and thus he controls the six seasonal changes and causes winter, summer, rain and so on. Yogīs and karmīs following the varṇāśrama institution, who practice haṭha or aṣṭāṅga-yoga or who perform agnihotra sacrifices, worship Sūrya Nārāyaṇa for their own benefit. The demigod Sūrya is always in touch with the Supreme Personality of Godhead, Nārāyaṇa. Residing in outer space, which is in the middle of the universe, between Bhūloka and Bhuvarloka, the sun rotates through the time circle of the zodiac, represented by twelve rāśis, or signs, and assumes different names according to the sign he is in. For the moon, every month is divided into two fortnights. Similarly, according to solar calculations, a month is equal to the time the sun spends in one constellation; two months constitute one season, and there are twelve months in a year. The entire area of the sky is divided into two halves, each representing an ayana, the course traversed by the sun within a period of six months. The sun travels sometimes slowly, sometimes swiftly and sometimes at a moderate speed. In this way it travels within the three worlds, consisting of the heavenly planets, the earthly planets and outer space. These orbits are referred to by great learned scholars by the names Saṁvatsara, Parivatsara, Iḍāvatsara, Anuvatsara and Vatsara.