information:highlighting
Early history

As early as the 14th century BC a complete list of the 36 Egyptian decans was placed among the hieroglyphs adorning the tomb of Seti I; they figured again in the temple of Ramesses II, and characterize every Egyptian astrological monument. Both the famous zodiacs of Dendera display their symbols, identified by Karl Richard Lepsius.

The division of the ecliptic into the zodiacal signs originates in Babylonian astronomy during the first half of the 1st millennium BC. The zodiac draws on stars in earlier Babylonian star catalogues, such as the MUL.APIN catalogue, which was compiled around 1000 BC. Some constellations can be traced even further back, to Bronze Age (Old Babylonian Empire) sources, including Gemini "The Twins", from Sumerian: 𒀯𒈦𒋰𒁀𒃲𒃲, romanized: MAŠ.TAB.BA.GAL.GAL "The Great Twins"; Cancer "The Crab", from Sumerian: 𒀯𒀠𒇻, romanized: AL.LUL "The Crayfish", among others.

Around the end of the fifth century BC, Babylonian astronomers divided the ecliptic into 12 equal "signs", by analogy to 12 schematic months of 30 days each. Each sign contained 30° of celestial longitude, thus creating the first known celestial coordinate system. According to calculations by modern astrophysics, the zodiac was introduced between 409 and 398 BC, during Persian rule, and probably within a very few years of 401 BC. Unlike modern astrologers, who place the beginning of the sign of Aries at the position of the Sun at the vernal equinox in the Northern Hemisphere (March equinox), Babylonian astronomers fixed the zodiac in relation to stars, placing the beginning of Cancer at the "Rear Twin Star" (β Geminorum) and the beginning of Aquarius at the "Rear Star of the Goat-Fish" (δ Capricorni).

Due to the precession of the equinoxes, the time of year the Sun is in a given constellation has changed since Babylonian times, as the point of March equinox has moved from Aries into Pisces.

Because the divisions were made into equal arcs of 30° each, they constituted an ideal system of reference for making predictions about a planet's longitude. However, Babylonian techniques of observational measurements were in a rudimentary stage of evolution. They measured the position of a planet in reference to a set of "normal stars" close to the ecliptic (±9° of latitude). The normal stars were used as observational reference points to help position a planet within this ecliptic coordinate system.

In Babylonian astronomical diaries, a planet position was generally given with respect to a zodiacal sign alone, though less often in specific degrees within a sign. When the degrees of longitude were given, they were expressed with reference to the 30° of the zodiacal sign, i.e., not with a reference to the continuous 360° ecliptic. In astronomical ephemerides, the positions of significant astronomical phenomena were computed in sexagesimal fractions of a degree (equivalent to minutes and seconds of arc). For daily ephemerides, the daily positions of a planet were not as important as the astrologically significant dates when the planet crossed from one zodiacal sign to the next.