Early understanding pulled by a horse is a sculpture believed to be illustrating an important part of
Nordic Bronze Age mythology.|alt=A sculpture of the sun in a chariot being pulled by a horse that has wheels instead of hoofs. In many prehistoric and ancient cultures, the Sun was thought to be a
solar deity or other
supernatural entity. In the early 1st millennium BC,
Babylonian astronomers observed that the Sun's motion along the
ecliptic is not uniform, though they did not know why; it is today known that this is due to the movement of Earth in an
elliptic orbit, moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion. One of the first people to offer a scientific or philosophical explanation for the Sun was the
Greek philosopher
Anaxagoras. He reasoned that it was a giant flaming ball of metal even larger than the land of the
Peloponnesus and that the Moon reflected the light of the Sun.
Eratosthenes estimated the distance between Earth and the Sun in the 3rd century BC as "of stadia
myriads 400 and 80000", the translation of which is ambiguous, implying either 4,080,000
stadia (755,000 km) or 804,000,000 stadia (148 to 153 million kilometres or 0.99 to 1.02 AU); the latter value is correct to within a few per cent. In the 1st century AD,
Ptolemy estimated the distance as 1,210 times
the radius of Earth, approximately . The theory that the Sun is the centre around which the planets orbit was first proposed by the ancient Greek
Aristarchus of Samos in the 3rd century BC, and later adopted by
Seleucus of Seleucia . This view was developed in a more detailed mathematical model of a heliocentric system in the 16th century by
Nicolaus Copernicus.
Development of scientific understanding 's |alt=A drawing of a man wearing a crown in a chariot, being pulled by horses. Observations of sunspots were recorded by
Chinese astronomers during the
Han dynasty (202 BCAD 220), with records of their observations being maintained for centuries.
Averroes also provided a description of sunspots in the 12th century. The invention of the telescope in the early 17th century permitted detailed observations of sunspots by
Thomas Harriot,
Galileo Galilei and other astronomers. Galileo posited that sunspots were on the surface of the Sun rather than small objects passing between Earth and the Sun.
Medieval Islamic astronomical contributions include
al-Battani's discovery that the direction of the Sun's
apogee (the place in the Sun's orbit against the fixed stars where it seems to be moving slowest) is changing. In modern heliocentric terms, this is caused by a gradual motion of the aphelion of the Earth's orbit.
Ibn Yunus observed more than 10,000 entries for the Sun's position for many years using a large
astrolabe. The first reasonably accurate distance to the Sun was determined in 1684 by
Giovanni Domenico Cassini. Knowing that direct measurements of the solar parallax were difficult, he chose to measure the Martian parallax. Having sent
Jean Richer to
Cayenne, part of
French Guiana, for simultaneous measurements, Cassini in Paris determined the parallax of
Mars when Mars was at its closest to Earth in 1672. Using the circumference distance between the two observations, Cassini calculated the Earth–Mars distance, then used
Kepler's laws to determine the Earth–Sun distance. His value, about 10% smaller than modern values, was much larger than all previous estimates. From an observation of a
transit of Venus in 1032,
Ibn Sina concluded that Venus was closer to Earth than the Sun. In 1677,
Edmond Halley observed a transit of Mercury across the Sun, leading him to realise that observations of the
solar parallax of a planet (more ideally using the transit of Venus) could be used to
trigonometrically determine the distances between Earth,
Venus, and the Sun. Observations of the
1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as , only 0.8% greater than the modern value. In 1666,
Isaac Newton observed the Sun's light using a
prism, and showed that it is made up of light of many colours. In 1800,
William Herschel discovered
infrared radiation beyond the red part of the solar spectrum. The 19th century saw advancement in spectroscopic studies of the Sun;
Joseph von Fraunhofer recorded more than 600
absorption lines in the spectrum, the strongest of which are still often referred to as
Fraunhofer lines. The 20th century brought about several specialised systems for observing the Sun, especially at different narrowband wavelengths, such as those using Calcium-H (396.9 nm), Calcium-K (393.37 nm) and
Hydrogen-alpha (656.46 nm)
filtering. During early studies of the
optical spectrum of the photosphere, some absorption lines were found that did not correspond to any
chemical elements then known on Earth. In 1868,
Norman Lockyer hypothesised that these absorption lines were caused by a new element that he dubbed
helium, after the Greek Sun god
Helios. Twenty-five years later, helium was isolated on Earth. In the early modern scientific era, the source of the Sun's energy was a significant puzzle.
Lord Kelvin suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat. Kelvin and
Hermann von Helmholtz proposed a
gravitational contraction mechanism to explain the energy output, but the resulting age estimate was only 20 million years, well short of the time span of at least 300 million years suggested by some geological discoveries of that time. In 1890, Lockyer proposed a meteoritic hypothesis for the formation and evolution of the Sun. Not until 1904 was a documented solution offered.
Ernest Rutherford suggested that the Sun's output could be maintained by an internal source of heat, and suggested
radioactive decay as the source.
Albert Einstein provided the essential clue to the source of the Sun's energy output with his
mass–energy equivalence relation . In 1920, Sir
Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass. The preponderance of hydrogen in the Sun was confirmed in 1925 by
Cecilia Payne using the ionisation theory developed by
Meghnad Saha. The theoretical concept of fusion was developed in the 1930s by the astrophysicists
Subrahmanyan Chandrasekhar and
Hans Bethe. Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun. In 1957,
Margaret Burbidge,
Geoffrey Burbidge,
William Fowler and
Fred Hoyle showed that most of the elements in the universe have been
synthesised by nuclear reactions inside stars, some like the Sun.
Solar space missions The first satellites designed for long-term observation of the Sun from interplanetary space were
Pioneer 6, 7, 8, and 9, launched by NASA between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field.
Pioneer 9 operated for a particularly long time, transmitting data until May 1983. In the 1970s, two
Helios spacecraft and the Skylab
Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The
Helios 1 and
2 probes were U.S.–German collaborations that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion. probe|alt=See caption In 1980, the
Solar Maximum Mission probes were launched by NASA. This spacecraft was designed to observe gamma rays,
X-rays and
ultraviolet radiation from solar flares during a time of high solar activity and solar luminosity. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent three years in this inactive state. In 1984,
Space Shuttle Challenger mission
STS-41-C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before
re-entering Earth's atmosphere in June 1989. Launched in 1991, Japan's
Yohkoh (
Sunbeam) satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares and demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005. The
Solar and Heliospheric Observatory, jointly built by the
European Space Agency and NASA, was launched on 2 December 1995. SOHO remains in operation as of 2024. Situated at the
Lagrangian point between Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch. testing at the vacuum spin-balancing facility|alt=A photograph of Ulysses spacecraft All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The
Ulysses probe was launched in 1990 to study the Sun's polar regions. It first travelled to Jupiter, to "slingshot" into an orbit that would take it far above the plane of the ecliptic. Once
Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s (slower than expected) and that there were large magnetic waves emerging from high latitudes that scattered galactic cosmic rays. Elemental abundances in the photosphere are well known from
spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission,
Genesis, was designed to allow astronomers to directly measure the composition of solar material. == Religious aspects ==