The word udaya means ‘rising of’ and asta means ‘setting of’ is an important concept in Hindu religion. The combined word, in a general context, refers to the rising and setting of the sun above the local horizon, leading to the division of the day and the night. The transit times are referred to as the ‘sandhya kala.’ At this time, the worship at temples has to be duly performed according to Agamas. This natural dynamic of a cosmic phenomenon with reference to the sun has been considered for the purpose of ascertaining the length of the day and night in astrology, leading to the correct assessment of the time of lagna/rashi pramana at any given point.
As for the other planets (graha-udaya and graham-asta), the rising and setting is considered with their proximity to the sun. Astrologically, the planets in their udaya and astra status give different types of result. In performing auspicious functions, the rising and setting of benefic planets like Guru and Shukra need to be considered.
When the sun is rising, the performance of certain acts like soliciting the company of women, intercourse, and drinking are prescribed as sinful activities. The rising of the sun above the mythical Meru mountain ad going beyond it is deemed to be the rise and set of sun in Puranas. There is prescription by Manu that the rising and setting sun should not be seen with the naked eye.