Celestial Longitude (Babylonian-Stratum)

Definition

Celestial longitude is how far a celestial body sits along the ecliptic — the Sun's yearly path — measured eastward from a chosen zero-point. It runs in degrees and minutes from 0° to 360°, and you can also write it as a sign plus degrees: 15° Leo is the same as 135° of absolute longitude. It is the main coordinate astrologers use to place a planet within a zodiac sign. Paired with celestial latitude — how far the body sits north or south of the ecliptic — it gives a full position in the ecliptic frame.

In Tradition

Read at the Babylonian stratum, Rochberg shows Babylonian astronomers recorded a planet's place in two coordinate systems. The first gave Normal-Star offsets in linear units — cubits of 2°, fingers of one-twelfth of a degree. The second gave longitude in the twelve-segment mathematical zodiac, written as a sign plus degrees within that sign; the late-Babylonian horoscopes BH 1 and BH 2, both 410 BCE, use it. The Greek tradition — Hipparchus, Ptolemy — kept this longitude convention and added the geometric celestial-sphere framework.

In Practice

For a historian tracing how the idea travelled, the Babylonian-stratum reading makes plain that the modern Western chart's way of writing a position — "Mars at 22°15' Scorpio" — descends directly from late-Babylonian horoscope-construction. A Babylonian astronomer found a planet's longitude by watching its offset from a Normal Star in cubits and fingers, converting that through the Normal Star's own known longitude, and rounding to a sign plus degrees. The mathematical-astronomy texts, System A and System B, instead computed longitude in advance, using arithmetic step-functions and zigzag-functions rather than any geometric model. Greek astronomy then added what was missing: a correction for the tilt of the ecliptic, the celestial-sphere model itself, and the latitude-longitude pairing needed for bodies that stray off the ecliptic.

Historical Origin

The Babylonian Normal-Star offset coordinate system is attested in the Astronomical Diaries (5th century BCE onward; Sachs-Hunger 1988-1996). The 12 × 30° zodiac longitude system is attested in late-5th-century-BCE diaries and the earliest horoscopes, BH 1 and BH 2, both 410 BCE (Rochberg 1998). The System A and System B procedure texts belong to the Hellenistic period (Neugebauer, Astronomical Cuneiform Texts 1955; Hunger-Pingree 1999). The Greek geometric-coordinate framework appears in Ptolemy's Almagest I-II and VII (2nd century CE).

Definition

In Tradition

In Practice

Historical Origin

Further Reading