Comets have been observed by humanity for thousands of years, but only in the past few centuries have they been studied as astronomical phenomena. Before modern times, great comets caused worldwide fear, considered bad omens foreboding disaster and turmoil; for example, the 1066 passage of Halley's Comet depicted as heralding the Norman conquest of England. As the science of astronomy developed planetary theories, understanding the nature and composition of comets became a challenging mystery and a large area of study.
Halley's Comet, reappearing every 75–76 years, was pivotal to the study of comets and their orbits. Thinkers such as Immanuel Kant in the eighteenth century hypothesized about the physical composition of comets. Today, comets are well understood as "dirty snowballs" in eccentric orbits around the Sun, but they continue to be objects of scientific and popular fascination. In 1994, comet Shoemaker–Levy crashed spectacularly into the atmosphere of Jupiter. In 1997, members of the Heaven's Gate cult committed mass suicide, inspired by the passage of comet Hale-Bopp. Since 1985, a total of 8 comets have been visited by spacecraft.
Early observations and thought
From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. Little is known of what people thought about comets before Aristotle, who observed his eponymous comet, and most of what is known comes secondhand. From cuneiform astronomical tablets, and works by Aristotle, Diodorus Siculus, Seneca, and one attributed to Plutarch but now thought to be Aetius, it is observed that ancient philosophers divided themselves into two main camps. Some believed comets to be astronomical entities; others affirmed their meteorological nature.
Meteors and comets were of great importance to the native inhabitants of Mexico. Meteors were alternatively viewed as arrows of stellar gods, as their cigar butts, and even as their excrement. The arrows could hit animals or people and were feared when walking at night. Comets were conceived as smoking stars and as bad omens, e.g., announcing the death of a ruler.
Ancient Chinese records of comet apparitions have been particularly useful to modern astronomers. They are accurate, extensive, and consistent over three millennia. The past orbits of many comets have been calculated entirely from these records and most notably they were used in connection with Halley's comet.Needham, Joseph, Science and Civilisation in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth, pp. 430–433, Cambridge University Press, 1959 . Ancient Chinese made important decisions by looking at celestial omens and comets were an important omen, always disastrous. Under the theory of Wu Xing (also known as five elements), comets were thought to signify an imbalance of yin and yang. Chinese emperors employed observers specifically to watch for them. Some important decisions were made as a result. For instance, Emperor Ruizong of Tang abdicated after a comet appearance in 712 CE. Comets were thought to have military significance. For example, the breakup of a comet on 35 CE was interpreted as portending the destruction of Gongsun Shu by Wu Han.
According to Norse Mythology, comets were actually a part of the Giant Ymir's skull. According to the tale, Odin and his brothers slew Ymir after the Battle of Ragnarok and set about constructing the world (Earth) from his corpse. They fashioned the oceans from his blood, the soil from his skin and muscles, vegetation from his hair, clouds from his brains, and the sky from his skull. Four dwarves, corresponding to the four cardinal directions, held Ymir's skull above the Earth. Following this tale, comets in the sky, as believed by the Norse, were flakes of Ymir's skull falling from the sky and then disintegrating.Alexander, Rachel. Myths, Symbols and Legends of Solar System Bodies by Rachel Alexander. 1st Ed. 2015.. ed. 2015. The Patrick Moore Practical Astronomy Ser., 177. Web.
The only place in the world where a comet is worshiped is in a temple at Rome. It was a comet that the divine Augustus judged as particularly propitious to himself since it appeared at the beginning of his rule during the games which he gave in honor of Venus Genetrix not long after his father's death when he was a member of the religious body that Caesar had founded.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky. Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way. Aristotle introduced his theory of how comets came to be by first stating that the world was divided into two parts: the earth and the heavens. The upper parts of the Earth, below the Moon, contained phenomena such as the Milky Way and comets. These phenomena were created from a mixture of four elements that were naturally found on Earth: water, earth, fire, and air. He theorized that the Earth was the center of the universe, which was surrounded by various other planets and stars. The universe or better known as the heavens filled the void above the terrestrial atmosphere with a fifth element called "Aether". Aristotle believed that comets were shooting stars that evolved into something much different. This proved that comets came from a combination of the elements found on Earth. Comets could not have come from the heavens as the heavens are never-changing but comets are ever-changing as they move through space. Aristotle believed that comets were shooting stars that evolved into something much different. Aristotle considered comets as a specific form of shooting stars that can occur under a very delicate combination of physical conditions. It is not known how many comet appearances Aristotle and his contemporaries witnessed or how much quantitative observational information they had about the trajectory, motion and duration of comets.
Anaxagoras and Democritus' theory deviated from Aristotle's, as they believed comets were only after-images or shadows from planetary eclipses. Pythagorean claimed comets were planets that revolved around the Sun for a longer period of time across the edge of the Sun. Hippocrates of Chios and Aeschylus had a similar belief to that of Pythagorean, as they both believe comets were planets that had special properties. Chios and Aeschylus theorized that comets are planets that have an immaterial tail produced by the atmosphere. Aristotle's theory over the creation and properties of a comet was prevalent up until the 1600s. Many philosophers and astrologers came up with their own theories to try and explain the phenomena that is a comet but only two were of relevance. Aristotle's theory still prevailed, along with Seneca's.
Seneca believed comets came from the celestial region of the universe. He firmly opposed Aristotle's theory that comets were formed from the element of fire by stating the comets fire would grow if it ever entered the lower depths of the atmosphere. Seneca recognized the flaws in his theory as he understood that accurately and consistently observing a comet had a high level of difficulty. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky.
Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. The earliest known picture of a comet is that of Halley's Comet depicted as a terrifying omen on the Bayeux Tapestry, which recorded the Norman conquest of England in 1066 CE. Another illustration published in the Nuremberg Chronicle in 1493 has been claimed to depict the comet's 684 CE apparition, but the same woodblock is used in that chronicle for other comets from the 11th century, and there is no evidence that the image is anything but a generic image of a comet, created for the 1493 edition.
Pre-Modern views on comets In the Islamicate empire, Nasir al-Din al-Tusi used the phenomena of comets to refute Ptolemy's claim that a stationary Earth can be determined through observation. Ali Qushji, in his Concerning the Supposed Dependence of Astronomy upon Philosophy, rejected Aristotelian physics and completely separated natural philosophy from astronomy. After observing comets, Ali Qushji concluded, on the basis of empirical evidence rather than speculative philosophy, that the moving Earth theory is just as likely to be true as the stationary Earth theory and that it is not possible to empirically deduce which theory is true.
In the mid 1500s, a mathematician by the name of Jean Pena opposed Aristotle's theory of comets by studying the physics and math behind the phenomena. He deducted that comets maintained its visual appearance, regardless of the view and angle in which is observed near the horizon of the Sun. Pena argued that the orientation and appearance of the comets were due to the physics of space. Pena claimed comets were at a farther distance from the Earth than the Moon as it would pass the Moon at a greater speed, due to the effects of Earth's gravity. The tail of a comet points toward the direction of the Sun as it is moving through space based on the laws of refraction. The comet's tail is composed of an air-like element that is transparent as it is seen in space but only when it is faced away from the Sun. The visibility of the tail is explained by solar rays reflecting off of the tail. The Laws of Refraction allows the human eye to visually see the tail of a comet in space at a different position than it truly is because of the reflection from the Sun.