ज्योतिषे विश्वासटिप्पन्यः

Ecliptic

Sun is always at 0 deg latitude. It’s longitude (determinable using background stars) is 0 deg (=0 RA) during march vernal equinox, 90 deg (= 6h RA) during summer solstice, 270 (= 18h RA) during winter solstice etc..
Latitudes are parallel to each other, longitudes pass through the same two poles.
The 0 deg and 180 deg longitude together form the equinoctial colure.

Angle between the ecliptic and the celestial equator (they meet at equinoctial points) is around 23 deg.
Among celestial equatorial system logitudes: equinoctial and solstical colures.
At poles, celestial equator = horizon. No seasonally visible stars = all stars visible at long winter night.
At equator, celestial pole is at horizon. Celestial equator is directly overhead.
During equinox day, sun appears to travel along the celestial equator. Looking from the poles, sun will be at the horizon then. From the equator, sun will be along the circle passing straight overhead.
Sun appears to go along other celestial lattitudes during other days. During northern solstice day, it will appear to travel along ~23 deg north celestial lattitude.

Equatorial Right ascension (RA)/ declination (dec) relative to spring/ vernal equinox point and celestial poles : intro.
The advantage of the equatorial coordinate system is that it expresses the position of a star or galaxy in a way that is independent of the observer’s position on Earth.
Finding and using the celestial equatorial (and diurnal) planes- KV18.

shukla-yajurvedasya shatapathabrAhmaNe: “एता ह वै प्राच्यै दिशो न च्यवन्ते | सर्वाणि ह वा अन्यानि नक्षत्राणि प्राच्यै दिशश्च्यवन्ते तत्प्राच्यामेवास्यैतद्दिश्याहितौ भवतस्तस्मात्कृत्तिकास्वादधीत”। Proper East. 90deg from north. Possible only if the star is on the celestial equator. Then the star will rise exactly at east. Krttikas on celestial equator around 2900BCE (two centuries before and after can be considered as acceptable)। [TW]

Aka “Polar longitude” (ecliptic longitude, but projection on meridian lines) in astro.com. Certain ayanAMsha-s are defined based on this system.