In an attempt to understand the will of the gods, the priests of Sumer began to study the stars, becoming the first professional astronomers. They did this from temples on top of pyramid-like structures called ziggurats. During their observations, they discovered all the planets visible to the unaided eye: Mercury, Venus, Mars, Jupiter, and Saturn. In time, they also began to notice that the planets follow mathematically predictable paths across the sky, which they interpreted as part of a cosmic order installed by the gods.
Fig. 96 – The ziggurat of Ur (21st century)
The first ziggurats of Mesopotamia date to the 4th millennium BC, predating the Egyptian pyramids by perhaps a thousand years. An early example is the Anu Ziggurat of Uruk, of which only the ruins remain today (see Fig. 73). The most famous ziggurat was built in Ur during the reign of King Ur-Nammu (21st century BC). Ur-Nammu used a new type of brick for his building projects. Since at least the eighth millennium BC, the Sumerians had used sun-dried bricks, but during his reign, the much more durable oven-baked bricks were discovered. The first level of the ziggurat of Ur still stands, although it was restored in the sixth century BC (see Fig. 96). In its original state, the entire structure is estimated to have had a height of around 30 meters.
In the past, every ziggurat was topped by a temple. Being closest to heaven, it became a place where priests contacted the gods. As a result, various ziggurats were called duranki, meaning “bond between heaven (an) and earth (ki).” The top of the ziggurat was also ideal for astronomical observations since it gave an unobstructed view of the sky. From here, teams of Sumerian priests maintained an unbroken watch of the night sky and systematically studied the heavens, making them the first professional astronomers.
After organizing the stars into fixed constellations, the astronomers of Sumer noticed that a few “stars” wandered from one constellation to the next. These special “stars” are now called planets. Mesmerized by this discovery, they made them a common theme in their art. Venus was often symbolized by an eight-pointed star, the moon by a crescent, and the sun by an equal-armed cross (see Fig. 97). In Fig. 98, we see a king wearing the symbol of the sun around his neck, demonstrating his connection to this divine sphere.
Fig. 97 – A king presenting his daughter to goddess Nannaya. Above we see Venus, the moon, and the sun (12th century BC) (Jastrow; Louvre, France)
Fig. 98 – King Shamshi-Adad V wearing the symbol of the sun around his neck. To his upper left we see Venus, the thunderbolt, the moon, a winged sun-disk, and the crown of the sky god Anu (c. 815 BC) (Osama Shukir Muhammed Amin, CC BY-SA 4.0; British Museum)
It was believed that each planet was animated by a god. Venus became associated with Inanna, the goddess of fertility, love, and war. The sun was associated with the god Shamash, who ruled over heaven and earth. The Moon became equated with the god Sin. This god was associated with the bull, perhaps because the crescent moon resembles the horns of a bull. Mars was associated with Nergal, the lord of the dead. Together with his wife, Ereshkigal, he ruled the underworld. Mars was often related to invasion, plague, and disease, possibly because of its red color. Jupiter was related to Marduk and was sometimes depicted as a spade. This planet signified good harvest, peace, and security.
One of the main tasks of the celestial gods was to decide fate, which they wrote down on a magical clay tablet called the Tablet of Destinies. As long as the supreme god Enlil controlled the tablet, he made sure that the universe was in order, but on various occasions, an evil being stole the tablet (see Fig. 99). Various planets became associated with the tablet. Saturn was equated with the conquering hero Ninurta, whose symbol was the eagle and who was believed to be the guardian of the tablet. Mercury was associated with Nabu, the god of the scribes (as a result, the symbol for this planet became the cuneiform wedge). When the gods assembled to decide the fate of the world for the next year, it was Nabu who recorded it on the tablet. Nabu was also considered to be the messenger of the gods, similar to the Roman god Mercury. This is most likely a reflection of the rapid motion of this planet.
Fig. 99 – Ninurta, the city god of Nimrud, with his thunderbolts, pursuing Anzu who stole the Tablet of Destinies from Enlil (c. 860 BC). (L. Gruner; British Museum)
In time, the Mesopotamian astronomers also traced the movement of the planets and found they followed predictable periodic paths. From this they concluded that the gods had established a mathematical cosmic order. By studying this order, the astronomers believed they could read the will of the gods. As a result, the Sumerians called the night sky the “Book of Heaven,” and the astronomers were set the task of reading this book to figure out how to best serve the gods in order to receive their praise and avoid their anger.
An astronomical occurrence that particularly frightened the people of Mesopotamia was an eclipse of the sun. It was perceived as a direct attack by the gods on the king. For the king to stay safe, he had to be replaced by a temporary substitute, who had to perform a ritual and then “go to his fate.” What this meant is made clear in the following fragment:
Damqi […] who had ruled Assyria, Babylonia and all the countries died with his queen […] as a substitute for the king. […] He went to his destiny for their rescue. We prepared the burial chamber. He and his queen have been decorated, treated, displayed, buried, and wailed over. The burnt offering has been burnt, all omens have been canceled and numerous [rituals], ceremonies, exorcistic rites, […] chants [and] scribal recitations have been performed in perfect manner. 
Fig. 100 – Redrawing of a cylinder seal impression (the original seal is lost) depicting what appear to be constellations, from Susa (c. 2500 BC) (Near Eastern Seals, D. Collon; Louvre, SB 6680)
The twelve “modern” constellations of the zodiac all had their origins in Mesopotamia. It is possible that some of them go back to 3200 BC, when we start to see depictions that would later become associated with the zodiac. Take for instance the cylinder seal impression depicted in Fig. 100, from about 2500 BC. Twice we see the sun, the moon, and Venus depicted as a set. On the upper left, we see a hunter with a bow aiming at a horned animal, which we would later recognize as Sagittarius and Capricorn. In the middle of the upper band we see a scorpion (Scorpio) and below it we see a lion (Leo) eating a bull (Taurus). On the top half, we see a goddess standing on a lion with weapons on her shoulders. This could be Inanna (compare with Fig. 80), who would later be associated with Venus. 
A similar case can be made for the Seal of Adda from 2300 BC (see Fig. 93). In this impression we not only see the sun god Shamash, the Venus goddess Inanna, the lion, a hunting god, and an eagle (Aquila), but also the god Enki with streams of water coming from his shoulders, an image both the Egyptians and the Greeks would later adopt for the constellation Aquarius.
In the next example (Fig. 101), from the 12th century BC, the link with the constellations is solid. We see the scorpion (Scorpio), the lion (Leo), the serpent (Hydra), the eagle (Aquila) the goat-fish (Capricorn), Jupiter depicted as a spade, Mercury as a wedge, and Mars as a panther. 
Fig. 101 – Astrological symbols on a boundary stone (12th century BC) (J. Hinke, 1907)
An astronomical text from the same time period called the Mul.Apin (about which we will learn more later in this chapter) lists the constellations of the zodiac. It correctly states that the moon passes through all these constellations every lunar month. We read:
The gods who stand in the path of the moon, through whose region the Moon in the course of a month passes and whom he touches: The Pleiades, the Bull of Heaven [Taurus], the Shepherd of Heaven [Orion], the Old Man [Perseus], the Crook [Auriga], the Great Twins [Gemini], the Crab [Cancer], the Lion [Leo], the Furrow [which in the same text is also associated with the grain goddess Shala, which later turned into Virgo], the Scales [Libra], the Scorpion [Scorpio], Pabilsag [Sagittarius], the Goat-fish [Capricorn], the Great One [Aquarius], the Tails of the Swallow [which became the fish in the late Babylonian era], and the Hired Man [later Aries]. 
A later set of tablets from about 200 BC, called the Seleucid Zodiac, shows Jupiter and Mercury depicted as stars, among the constellations Hydra, Leo, Corvus (the crow), and Virgo.
Fig. 102 – Two parts of the Seleucid zodiac tablets showing from left to right – Jupiter, hydra, leo, corvus, Mercury, and Virgo (c. 200 BC) (Left: Vorderasiatisches Museum, Berlin; right: F. Thureau-Dangin, 1922; Louvre, France)
The only astronomical texts that have survived from the third millennium BC is a list of some 25 stars and a number of fragments on astrology, which include references to the planets. An early example from 2300 BC reads:
When the planet Venus [...] an omen of Sargon, of the king of the four quarters.
In a fragment from about 2000 BC, we read:
When the planet Jupiter turns his face when rising towards the west and you can see the face of the sky, and no wind blows, there will be famine and disaster will rule. 
When the Babylonians took over Mesopotamia, astronomical research intensified. Around the 17th century BC, the famous Venus Tablet of Ammisaduqa—the world’s oldest known astronomical text—was written. It gives a systematic compilation of omens drawn from the dates on which Venus first and last appeared above the horizon over a period of 21 years. The text also contains divinations about floods, the availability of food, the outcome of wars, and so on. For example:
In month Arahsamna, 11th day, Venus disappeared in the east. Two months […] days it stayed away in the sky. In month Tebeti, […] day, Venus became visible in the west: the harvest of the land will prosper. 
An even greater source of astronomical knowledge was found in the remains of the Royal Library of Nineveh, where a staggering 30,000 tablets on various topics were discovered. The library was founded by the Assyrian philosopher-king Ashurbanipal (669–631 BC), who had ordered his servants to collect texts from all around Mesopotamia for his library:
The rare tablets on your route that are not found in Assyria, seek out and bring to me. 
Ashurbanipal’s library contained a copy of the Enuma Anu Enlil, which was originally composed at some point in the second millennium BC and contained about 7000 omens on 70 tablets based on observations of lunar eclipses, the phases of the moon, the sun, solar eclipses, the planets, the constellations, shooting stars, some fixed stars, and various meteorological events such as fog, thunder, clouds, and storms.
Fig. 103 – Part of a star chart, called a planisphere (c. 1100 BC) These charts list stars that appear on the horizon each month. It also gives a number related to the length of the day in each month. This number was found by measuring the amount of water that ran out of a water clock during either day or night. (BabelStone, CC BY-SA 3.0; British Museum)
Fig. 104 – Photograph showing four planets in the same plane known as the ecliptic (from the bottom up: Mercury, Venus, Jupiter, Saturn). The sun and the moon also move on this plane.
Another important text from the library is the Mul.Apin, probably created around the turn of the first millennium. It consists of two tablets that contain a summation of all astronomical knowledge available at the time. By this point, astrologers had already discovered several long-term periodic movements of the sun, moon, planets, and stars, thus allowing them to roughly predict their future positions. We also read about the seasons, the equinoxes (when day and night are about equal length), the solstices (the shortest and longest days of the year), the visibility of the moon, the length of the day, the changing length of shadows during the year, and more. We are also given a list of stars on “the path of the moon.” The text then goes on to explain that the sun and other planets also move along this path. This is the first mention of the ecliptic in history. We now know the ecliptic to be the plane of the solar system in which all the planets move around the sun (see Fig. 104).
Around 500 BC, the Neo-Babylonians discovered that 235 lunar months are about identical to 19 solar years. If we divide 19 years by 235, we find the lunar month only seconds apart from the modern value. Around the same time, they also discovered the Saros cycle of 223 lunar months (about 18 years), after which the sun, the earth, and the moon return to roughly the same configuration. As a result, chances are high that eclipses recur after this interval.
The Persians, who became the dominant force in Mesopotamia from the sixth century BC, are credited with the discovery of the synodic and sidereal periods of the planets. The synodic period is the time between consecutive conjunctions of a planet with the sun. The sidereal period is the time that it takes a planet to pass through the entire twelve signs of the zodiac and return to its starting point. For instance, Saturn has a sidereal period of 29.5 years. Two of these periods are equal to 57 synodic periods. Similar accurate periods for all the planets were fixed early in the Persian era and were used to calculate the approximate future planetary positions.
Since the purpose of the Mesopotamian astrologers had always been to read the will of the gods, there was little motivation to go beyond a mere description of the motion of the planets. For the Greeks, this would no longer be enough. They wanted a rational mechanism to explain why the planets moved the way they did. We will read about their discoveries in a later chapter.