Sunrise and sunset
There are three main ways that a sunrise can be defined:
- the Sun’s upper limb is visible on the horizon;
- the center of the Sun’s disk is visible on the horizon;
- the center of the Sun’s disk is on the horizon, without considering refraction.
These options are listed in the order they happen chronologically. Initially, the Sun’s upper limb becomes visible on the horizon. Then, the center of the Sun’s disk becomes visible. Finally, the center of the Sun’s disk actually rises on the horizon, because the Sun becomes visible before it rises geometrically due to refraction.
Regarding option (1), there is an email by Pt. Sanjay Rath from 2002, in which he refers to Varahamihira as supporting this option. Unfortunately, no reference to an exact source is given.
Regarding option (2), a verse that is usually quoted is Bṛhat Pārāśara Horā Śāstra 9.14. In that verse, a definition of sandhyā is given based on the half of the Sun’s disk. The English translation and commentary by Girish Chand Sharma (1999) uses words “appearance” and “disappearance” when referring to the half of the Sun’s disk, and the work by R. Santhanam (1984) uses “the sight of the semi disc (half)”. Thus, the respected commentators make it clear that the visual definition of sandhyā is meant in this verse, rather than geometric definition. However, this verse gives the definition of sandhyā, and it is unclear whether a definition of sunrise for astrological calculations should be inferred from it.
Regarding option (3), this is a nice definition from a logical point of view, because with this definition the degrees of lagna and Sūrya (as well as special lagnas) coincide at sunrise. It is also logical, because it does not depend on the actual visibility of the Sun and can thus be computed without knowing the exact atmospheric conditions on a given day. However, while logical, we currently do not have a classical reference to substantiate this point of view.
Chakra Darshana uses option (1) for historical reasons. This topic is very important and requires further research, because many other calculations depend on it (either directly or indirectly): Abhijit muhūrta, Brahma muhūrta, bhāva lagna, horā lagna, ghaṭī lagna, varṇada lagna, prāṇapada lagna, Gulika, Gulika kāla, Rāhu kāla, Yamaghaṇṭaka, kāla horās, and others.