The history of Mars observation is about the recorded history of observation of the planet Mars. Some of the early records of Mars's observation date back to the era of the ancient Egyptian astronomers in the 2nd millennium BCE. Chinese records about the motions of Mars appeared before the founding of the Zhou dynasty (1045 BCE). Detailed observations of the position of Mars were made by Babylonian astronomers who developed arithmetic techniques to predict the future position of the planet. The ancient Greek philosophers and Hellenistic astronomers developed a geocentric model to explain the planet's motions. Measurements of Mars's angular diameter can be found in ancient Greek and Indian texts. In the 16th century, Nicolaus Copernicus proposed a heliocentric model for the Solar System in which the planets follow circular orbits about the Sun. This was revised by Johannes Kepler, yielding an elliptic orbit for Mars that more accurately fitted the observational data.
The first telescopic observation of Mars was by Galileo Galilei in 1609. Within a century, astronomers discovered distinct albedo features on the planet, including the dark patch Syrtis Major Planum and polar ice caps. They were able to determine the planet's rotation period and axial tilt. These observations were primarily made during the time intervals when the planet was located in opposition to the Sun, at which points Mars made its closest approaches to the Earth. Better telescopes developed early in the 19th century allowed permanent Martian albedo features to be mapped in detail. The first crude map of Mars was published in 1840, followed by more refined maps from 1877 onward. When astronomers mistakenly thought they had detected the spectroscopic signature of water in the Martian atmosphere, the idea of life on Mars became popularized among the public. Percival Lowell believed he could see a network of artificial canals on Mars. These linear features later proved to be an optical illusion, and the atmosphere was found to be too thin to support an Earth-like environment.
Yellow clouds on Mars have been observed since the 1870s, which Eugène M. Antoniadi suggested were windblown sand or dust. During the 1920s, the range of Martian surface temperature was measured; it ranged from . The planetary atmosphere was found to be arid with only trace amounts of oxygen and water. In 1947, Gerard Kuiper showed that the thin Martian atmosphere contained extensive carbon dioxide; roughly double the quantity found in Earth's atmosphere. The first standard nomenclature for Mars albedo features was adopted in 1960 by the International Astronomical Union. Since the 1960s, multiple robotic spacecraft have been sent to explore Mars from orbit and the surface. The planet has remained under observation by ground and space-based instruments across a broad range of the electromagnetic spectrum. The discovery of meteorites on Earth that originated on Mars has allowed laboratory examination of the chemical conditions on the planet.
Earliest records
The existence of Mars as a wandering object in the night sky was recorded by ancient Egyptian astronomers. By the 2nd millennium BCE they were familiar with the apparent retrograde motion of the planet, in which it appears to move in the opposite direction across the sky from its normal progression. Mars was portrayed on the ceiling of the tomb of Seti I, on the Ramesseum ceiling, and in the Senenmut star map. The last is the oldest known star map, being dated to 1534 BCE based on the position of the planets.
By the period of the Neo-Babylonian Empire, Babylonian astronomers were making systematic observations of the positions and behavior of the planets. For Mars, they knew, for example, that the planet made 37 synodic periods, or 42 circuits of the zodiac, every 79 years. The Babylonians invented arithmetic methods for making minor corrections to the predicted positions of the planets. This technique was primarily derived from timing measurements—such as when Mars rose above the horizon, rather than from the less accurately known position of the planet on the celestial sphere.
Chinese records of the appearances and motions of Mars appear before the founding of the Zhou dynasty (1045 BCE), and by the Qin dynasty (221 BCE) astronomers maintained close records of planetary conjunctions, including those of Mars. Occultations of Mars by Venus were noted in 368, 375, and 405 CE. The period and motion of the planet's orbit was known in detail during the Tang dynasty (618 CE).
The early astronomy of ancient Greece was influenced by knowledge transmitted from the Mesopotamian culture. Thus, the Babylonians associated Mars with Nergal, their god of war and pestilence, and the Greeks connected the planet with their god of war, Ares. During this period, the motions of the planets were of little interest to the Greeks; Hesiod's Works and Days (c. 650 BCE) makes no mention of the planets.
Orbital models
The Greeks used the word planēton to refer to the seven celestial bodies that moved with respect to the background stars and they held a geocentric view that these bodies moved about the Earth. In his work, The Republic (X.616E–617B), the Greek philosopher Plato provided the oldest known statement defining the order of the planets in Greek astronomical tradition. His list, in order of the nearest to the most distant from the Earth, was as follows: the Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and the fixed stars. In his dialogue Timaeus, Plato proposed that the progression of these objects across the skies depended on their distance, so that the most distant object moved the slowest.
Aristotle, a student of Plato, observed an occultation of Mars by the Moon on 4 May 357 BCE. From this he concluded that Mars must lie further from the Earth than the Moon. He noted that other such occultations of stars and planets had been observed by the Egyptians and Babylonians.In China, astronomers recorded an occultation of Mars by the Moon in 69 BCE. See Price (2000:148). Aristotle used this observational evidence to support the Greek sequencing of the planets. His work De Caelo presented a model of the universe in which the Sun, Moon, and planets circle about the Earth at fixed distances. A more sophisticated version of the geocentric model was developed by the Greek astronomer Hipparchus when he proposed that Mars moved along a circular track called the epicycle that, in turn, orbited about the Earth along a larger circle called the deferent.
In Roman Egypt during the 2nd century CE, Claudius Ptolemaeus (Ptolemy) attempted to address the problem of the orbital motion of Mars. Observations of Mars had shown that the planet appeared to move 40% faster on one side of its orbit than the other, in conflict with the Aristotelian model of uniform motion. Ptolemy modified the model of planetary motion by adding a point offset from the center of the planet's circular orbit about which the planet moves at a uniform rate of rotation. He proposed that the order of the planets, by increasing distance, was: the Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, and the fixed stars. Ptolemy's model and his collective work on astronomy was presented in the multi-volume collection Almagest, which became the authoritative treatise on Western astronomy for the next fourteen centuries.
In 1543, Nicolaus Copernicus published a heliocentric model in his work De revolutionibus orbium coelestium. This approach placed the Earth in an orbit around the Sun between the circular orbits of Venus and Mars. His model successfully explained why the planets Mars, Jupiter and Saturn were on the opposite side of the sky from the Sun whenever they were in the middle of their retrograde motions. Copernicus was able to sort the planets into their correct heliocentric order based solely on the period of their orbits about the Sun. His theory gradually gained acceptance among European astronomers, particularly after the publication of the Prutenic Tables by the German astronomer Erasmus Reinhold in 1551, which were computed using the Copernican model.
On October 13, 1590, the German astronomer Michael Maestlin observed an occultation of Mars by Venus. One of his students, Johannes Kepler, quickly became an adherent to the Copernican system. After the completion of his education, Kepler became an assistant to the Danish nobleman and astronomer, Tycho Brahe. With access granted to Tycho's detailed observations of Mars, Kepler was set to work mathematically assembling a replacement to the Prutenic Tables. After repeatedly failing to fit the motion of Mars into a circular orbit as required under Copernicanism, he succeeded in matching Tycho's observations by assuming the orbit was an ellipse and the Sun was located at one of the foci. His model became the basis for Kepler's laws of planetary motion, which were published in his multi-volume work Epitome Astronomiae Copernicanae (Epitome of Copernican Astronomy) between 1615 and 1621.
Early telescope observations At its closest approach, the angular size of Mars is 25 arcseconds (a unit of degree); this is much too small for the naked eye to resolve. Hence, prior to the invention of the telescope, nothing was known about the planet besides its red hue and its position on the sky. Telescopes appeared in 1608 and in September 1610 the Italian scientist Galileo Galilei indicated in his records that he began observing Mars through a telescope. This instrument was too primitive to display any surface detail on the planet, so he set the goal of seeing if Mars exhibited phases of partial darkness similar to Venus or the Moon. Although uncertain of his success, by December he did note that Mars had shrunk in angular size. Polish astronomer Johannes Hevelius succeeded in observing a phase of Mars in 1645.
In 1636 Francesco Fontana drew the first telescopic observation of Mars, but noted no actual features. In 1644, the Italian Jesuit Daniello Bartoli reported seeing two darker patches on Mars. During the oppositions of 1651, 1653 and 1655, when the planet made its closest approaches to the Earth, the Italian astronomer Giovanni Battista Riccioli and his student Francesco Maria Grimaldi noted patches of differing reflectivity on Mars. The first person to draw Mars with observed actual features was the Dutch astronomer Christiaan Huygens. On November 28, 1659, he made an illustration of Mars that showed the distinct dark region now known as Syrtis Major Planum, and possibly one of the polar ice caps. The same year, he succeeded in measuring the rotation period of the planet, giving it as approximately 24 hours. He made a rough estimate of the diameter of Mars, guessing that it is about 60% of the size of the Earth, which compares well with the modern value of 53%. Perhaps the first definitive mention of Mars's southern polar ice cap was by the Italian astronomer Giovanni Domenico Cassini, in 1666. That same year, he used observations of the surface markings on Mars to determine a rotation period of 24h 40m. This differs from the currently-accepted value by less than three minutes. In 1672, Huygens noticed a fuzzy white cap at the north pole.