Question

When does 'Viṣuva-dina' (Equinox day, when day and night are equal) occur according to the 'Sāyaṇa' (tropical) system of Indian Astronomy?
A. Sāyaṇa Mēṣa Saṃkrānti (Sun enters tropical Aries)
B. Sāyaṇa Karka Saṃkrānti (Sun enters tropical Cancer)
C. Sāyaṇa Tulā Saṃkrānti (Sun enters tropical Libra)
D. Sāyaṇa Makara Saṃkrānti (Sun enters tropical Capricorn)
E. Sāyaṇa Kanyā Saṃkrānti (Sun enters tropical Virgo)

• A. Only B, D
• B. Only A, C
• C. Only D, E
• D. Only C, D

Hint:

Remember: Equinox = Equality. Day = Night. This happens at the 1st sign (Aries) and the 7th sign (Libra). These are the two points where the Sun's path crosses the "Middle line" (Equator).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
'Viṣuva' means 'Equinox'. It is the moment when the Sun's declination is exactly zero degrees, as it crosses the celestial equator. On this day, the duration of day and night is equal (12 hours or 30 Ghaṭis each) everywhere on Earth. In the 'Sāyaṇa' system (which accounts for the precession of equinoxes), these points are fixed at the entry points of the signs Aries and Libra.

Step 2: Detailed Explanation:
In spherical astronomy, the 'Krānti-vr̥tta' (Ecliptic) and 'Nāḍī-vr̥tta' (Celestial Equator) intersect at two points. These are the Equinoxes:

1. Sāyaṇa Mēṣa Saṃkrānti (A): This is the Vernal Equinox (Spring Equinox), occurring around March 21st. The Sun moves from the Southern Hemisphere to the Northern Hemisphere. Declination is $0^{\circ}$. This is 'Viṣuva'.
2. Sāyaṇa Tulā Saṃkrānti (C): This is the Autumnal Equinox, occurring around September 23rd. The Sun moves from the Northern Hemisphere to the Southern Hemisphere. Declination is again $0^{\circ}$. This is also 'Viṣuva'.

Contrast with other options:
- Sāyaṇa Karka (B): This is the Summer Solstice (longest day in the North).
- Sāyaṇa Makara (D): This is the Winter Solstice (shortest day in the North).
- Sāyaṇa Kanyā (E): This is just a regular monthly transit with no special astronomical 'Equinox' status.

The term 'Viṣuva' signifies balance. Ancient Indian astronomers used these days to calibrate their 'Śaṅku' (gnomon) and determine the local 'Palabhā'. While the 'Nirayaṇa' system used for horoscopes has shifted away from these points due to 'Ayanāṃśa', the 'Viṣuva' days are strictly defined by the 'Sāyaṇa' entry of Sun into Aries and Libra.

Step 3: Final Answer:
The Equinoxes occur during Sāyaṇa Mēṣa and Sāyaṇa Tulā Saṃkrānti (A and C).