Hora from Ahoratra — Etymology & the Seven Visible Planets

The Sanskrit word "Ahoratra" is a compound of "Aha" (day) and "Ratra" (night) — it denotes one complete cycle of daylight and darkness. By a well-attested etymological process, the first syllable (a-) and the last portion (-ratra) were dropped, yielding the middle portion: "hora." This word entered Greek as "hora" (ὥρα), passed through Latin, and eventually became the English "hour." Whether the borrowing went from India to Greece or vice versa is debated, but the linguistic kinship is undeniable.

A hora is 1/24th of an Ahoratra — one planetary hour. Each hora is ruled by one of the seven classical "planets" (grahas) visible to the naked eye. These seven are ranked by their apparent orbital period as seen from Earth, from slowest to fastest: Saturn (~29.5 years), Jupiter (~12 years), Mars (~2 years), Sun (~1 year), Venus (~225 days), Mercury (~88 days), Moon (~27 days). This ranking is called the Chaldean order, after the Babylonian astronomers who first systematized it.

Crucially, Rahu and Ketu are excluded from this system. They are not visible celestial bodies — they are mathematical points (lunar nodes) where the Moon's orbital plane intersects the ecliptic. While they are central to Indian astrology as part of the Navagraha (nine planets), they play no role in the hora-weekday derivation. Only the seven bodies observable with the naked eye participate in the Chaldean sequence.