The Sun is a yellow dwarf star at the center of our Solar System. Energy from the Sun, in the form of sunlight, supports almost all life on Earth and regulates the Earth’s climate and weather.
Sunlight is Earth’s primary source of energy. Photosynthesis by plants captures the energy of sunlight and converts it to chemical form, while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work. The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.
Humanity’s most fundamental understanding of the Sun is as the luminous disk in the sky, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural phenomenon.
Worship of the Sun was central to civilizations such as the Inca of South America and the Aztecs of what is now Mexico. Many ancient monuments were constructed with solar phenomena in mind. Stone megaliths accurately mark the summer or winter solstice. Some of the most prominent megaliths are located in Nabta Playa in Egypt and at Stonehenge in England. The pyramid of El Castillo in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumn equinoxes.
During the Roman era the winter solstice was a holiday celebrated as Sol Invictus which is an antecedent to Christmas. With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so Greek astronomers considered it to be one of the seven planets, after which the seven days of the week are named in some languages.
Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Ultraviolet light is strongly attenuated by Earth’s ozone layer, so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in human skin color in different regions of the globe.
Observed from Earth, the Sun’s path across the sky varies throughout the year. While the most obvious variation in the Sun’s apparent position through the year is a north south swing over 47 degrees of angle, there is an east west component as well, caused by the acceleration of the Earth as it approaches its perihelion with the Sun, and the reduction in the Earth’s speed as it moves away to approach its aphelion. The north south swing in apparent angle is the main source of seasons on Earth.
A rare optical phenomenon may occur shortly after sunset or before sunrise, known as a green flash. The flash is caused by light from the sun just below the horizon being bent, usually through a temperature inversion, towards the observer. Light of shorter wavelengths, such as violet, blue and green, is bent more than that of the longer wavelengths yellow, orange and red, but the violet and blue light is scattered more, leaving light that is perceived as green.
Sunlight is very bright, and looking directly at the Sun with the naked eye for brief periods can be painful, but is not particularly hazardous for normal eyes. Looking directly at the Sun causes visual artifacts and temporary partial blindness. Long-duration viewing of the direct Sun with the naked eye can begin to cause sunburn-like lesions on the retina after about 100 seconds, particularly under conditions where the light from the Sun is intense and well focused.
Partial solar eclipses are hazardous to view because the eye’s pupil is not adapted to the unusually high visual contrast. The pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. In the overall gloom, the pupil expands, and each retinal cell exposed to the solar image receives about ten times more light than it would looking at the non-eclipsed Sun. This can damage or kill those cells, resulting in small permanent blind spots for the viewer.