Exploration of The Moon - Early History

Early History

In Mesopotamia, Babylonian astronomers by the early first millennium BC had discovered a repeating 18-year cycle of lunar eclipses. They had also known by this time that 19 solar years is about equal to 235 lunar months. In the 2nd century BC, Seleucus of Seleucia correctly theorized that tides were caused by the Moon, although he believed that the interaction was mediated by the Earth's atmosphere. According to Strabo (1.1.9), Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.

By the mid-first millennium BC, Indian astronomers described the Moon’s monthly elongation in the Aitareya Brāhmana. By 499 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause behind the shining of the moon.

The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile. In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether. This separation was held to be part of Aristotelian physics for many centuries after. In the philosophy of Aristotle, the heavens, starting at the Moon, were the realm of perfection, the sublunary region was the realm of change and corruption, and any resemblance between these regions was strictly ruled out. Aristotle himself suggested that the Moon partook perhaps of some contamination from the realm of corruption. In his little book On the Face in the Moon's Orb, Plutarch expressed rather different views on the relationship between the Moon and Earth. He suggested that the Moon had deep recesses in which the light of the Sun did not reach and that the spots are nothing but the shadows of rivers or deep chasms. He also entertained the possibility that the Moon was inhabited. It had been suggested already in antiquity that the Moon was a perfect mirror and that its markings were reflections of earthly features, but this explanation was easily dismissed because the face of the Moon never changes as it moves about the Earth. The explanation that finally became standard was that there were variations of "density" in the Moon that caused this otherwise perfectly spherical body to appear the way it does. The perfection of the Moon, and therefore the heavens, was thus preserved. Aristarchus went a step further and computed the distance from Earth, together with its size, obtaining a value of 20 times the Earth radius for the distance (the real value is 60; the Earth radius was roughly known since Eratosthenes).

During the Warring States of China, astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses based on the relative positions of the Moon and Sun. Although the Chinese of the Han Dynasty (202 BC–202 AD) believed the Moon to be energy equated to qi, their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun (mentioned by Anaxagoras above). This was supported by mainstream thinkers such as Jing Fang (78–37 BC) and Zhang Heng (78–139 AD), but it was also opposed by the influential philosopher Wang Chong (27–97 AD). Jing Fang noted the sphericity of the Moon, while Zhang Heng accurately described a lunar eclipse and solar eclipse. These assertions were supported by Shen Kuo (1031–1095) of the Song Dynasty (960–1279) who created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. He also noted that the reason for the Sun and Moon not eclipsing every time their paths met was because of a small obliquity in their orbital paths.

Habash al-Hasib al-Marwazi, a Persian astronomer, conducted various observations at the Al-Shammisiyyah observatory in Baghdad between 825 and 835 AD. Using these observations, he estimated the Moon's diameter as 3,037 km (equivalent to 1,519 km radius) and its distance from the Earth as 215,209 miles, which come close to the currently accepted values. In 1021, the Islamic physicist, Alhazen, accurately explained the Moon illusion in the Book of Optics, which stated that judging the distance of an object depends on there being an uninterrupted sequence of intervening bodies between the object and the observer. With the Moon, there are no intervening objects, therefore since the size of an object depends on its observed distance, which is in this case inaccurate, the Moon appears larger on the horizon. Through Alhazen's work, the Moon illusion gradually came to be accepted as a psychological phenomenon. He also investigated moonlight, which he proved through experimentation that it originates from sunlight and correctly concluded that it "emits light from those portions of its surface which the sun's light strikes."

By the Middle Ages, before the invention of the telescope, an increasing number of people began to recognise the Moon as a sphere, though many believed that it was "perfectly smooth". In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had mountains and craters. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today. On maps, the dark parts of the Moon's surface were called maria (singular mare) or seas, and the light parts were called terrae or continents.

The medieval followers of Aristotle, in the Islamic world and then in Christian Europe, tried to make sense of the lunar spots in Aristotelian terms. Thomas Harriot, as well as Galilei, drew the first telescopic representation of the Moon and observed it for several years. His drawings, however, remained unpublished. The first map of the Moon was made by the Belgian cosmographer and astronomer Michael Florent van Langren in 1645. Two years later a much more influential effort was published by Johannes Hevelius. In 1647 Hevelius published Selenographia, the first treatise entirely devoted to the Moon. Hevelius's nomenclature, although used in Protestant countries until the eighteenth century, was replaced by the system published in 1651 by the Jesuit astronomer Giovanni Battista Riccioli, who gave the large naked-eye spots the names of seas and the telescopic spots (now called craters) the name of philosophers and astronomers. In 1753 the Croatian Jesuit and astronomer Roger Joseph Boscovich discovered the absence of atmosphere on the Moon. In 1824 Franz von Gruithuisen explained the formation of craters as a result of meteorite strikes.

The possibility that the Moon contains vegetation and is inhabited by selenites was seriously considered by major astronomers even into the first decades of the 19th century. The contrast between the brighter highlands and darker maria create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others. In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon. Almost at the same time however (during 1834–1836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume Mappa Selenographica and the book Der Mond in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere.