Sandhyā

Astronomy

verifiableSunrise, sunset and twilight come from the NOAA solar-position algorithm (Meeus, Astronomical Algorithms), computed in your browser — no network call, no almanac lookup. Sunrise and sunset use a solar zenith of 90.833°, which folds in the standard 34′ of atmospheric refraction plus the Sun's 16′ semi-diameter, so the instant is upper limb touching the horizon. Expected error is under a minute at low and mid latitudes.

verifiableTransit is true local solar noon — the Sun on your meridian. It drifts from clock noon by the equation of time (up to ±16 min) plus your longitude offset from the timezone's standard meridian (4 min per degree). In Goa that offset alone is about +26 min behind IST; in Kochi about +35 min.

verifiableOne ghaṭikā is 24 minutes — sixty of them make an ahorātra of 24 hours.

What is attested in the texts

attestedManusmṛti 2.101 fixes the two endpoints by posture: the morning Sāvitrī is recited standing, until the Sun appears; the evening one seated, until the stars become clearly visible. Every boundary this app draws for prātaḥ and sāyam is derived from that one rule, not from a clock.

attestedThe three-fold grading — uttama / madhyama / adhama — is standard in the practical manuals. That the grading exists is well attested. The exact clock boundaries are not uniform across sampradāyas, which is why they are adjustable above.

attestedThe daytime's five-fold division — prātaḥ, saṅgava, madhyāhna, aparāhṇa, sāyāhna — is standard Dharmaśāstra. Madhyāhna is the third part.

Choices I made — treat these as inference, not doctrine

• "Stars visible" is mapped to nautical twilight, the Sun 12° below the horizon, when most stars are still or already out. Civil twilight (6°) is the alternative and is offered in the settings. No text specifies a degree — this is a modern proxy for a naked-eye test.
• Aruṇodaya is taken as 4 ghaṭikās before sunrise. This is the figure most often cited, but 2 and 3 are also used, so it is a setting.
• Grades are assigned by start time, with a separate concluding target, because that is what the Manusmṛti rule actually constrains. A session that begins in uttama and runs past sunrise is still uttama.
• The adhama tail is cut off arbitrarily — one ghaṭikā past sunrise in the morning, six degrees past the star threshold in the evening. Nothing turns on those endpoints.

What this does not do

limitationNo pañcāṅga elements — no tithi, nakṣatra, yoga or karaṇa, and no vāra-based adjustment. A dṛk-gaṇita pañcāṅga will usually agree on sunrise to within a minute or two, and may disagree materially on window boundaries wherever your tradition's rule differs from the ones above.

limitationTimes are drawn for a flat sea-level horizon. Hills, buildings or elevation will shift the sun you actually see by several minutes — later at rising, earlier at setting, if the western horizon is high.

limitationClock times are rendered in your device's timezone. If you set coordinates far from where the device thinks it is, the instants remain correct but the labels will look wrong; a warning appears above when the mismatch is large.

Etymology

• सन्ध्या sandhyā — from sam + √dhā, "to place together": the joint, the seam where day and night are fastened.
• अरुणोदय aruṇodaya — the rising of Aruṇa, the Sun's charioteer, who goes ahead of him; the reddening before the disc.
• घटिका ghaṭikā — "little pot," after the pierced copper bowl sunk in water that took exactly this long to fill.
• माध्याह्निक mādhyāhnika — "belonging to midday," from madhya (middle) + ahan (day).