Earth-Sun Relations

The Sun is the center of the solar system and supports all life on Earth. It affects the weather, climate, and seasons; it drives the food chain through photosynthesis; and it provides heat, light, and energy. Ultimately, the Sun's demise will bring about the end of Earth.

Overview

The Sun was born 4.57 billion years ago out of the collapse of a portion of a stellar nursery, a molecular cloud of enormous density and size. The giant, hot sphere of plasma and gas is the center of the solar system and the supporter of all life on Earth.

The Sun accounts for approximately 99.86 percent of the mass of the solar system; its total mass is 2 × 10³⁰ kilograms (about 330,000 times the mass of Earth). Its diameter is roughly 1.4 million kilometers (more than one hundred times that of Earth), and its surface temperature is estimated to be 5,778 kelvin (5,505 degrees Celsius or 9,940 degrees Fahrenheit), while its core temperature is estimated at 1.571 × 10⁷ kelvin. Meanwhile, on Earth, the maximum surface temperature is around 331 kelvins. Three-fourths of the Sun is made of hydrogen, and nearly one-fourth is made of helium. Trace amounts of heavier elements make up the rest.

The Sun is the closest star to Earth and, thus, the brightest to appear in the sky. While the distance between the Sun and the Earth is constantly changing based on the Earth’s 365.25-day orbit around the Sun and based on the Earth’s twenty-four-hour rotation around its own axis, the mean distance between the Sun and the Earth is 149.6 million km (93 million miles), a distance that defines one astronomical unit. It takes light from the Sun eight minutes and nineteen seconds to cover this distance and reach Earth. For comparison, consider the 1.3 seconds it takes light to reach the Earth from the Moon, or, on the other end of the spectrum, the one hundred thousand years it takes to cross the Milky Way galaxy.

The Sun affects the Earth in a multitude of ways. Its complex magnetic field causes a range of effects known as solar activity, including sunspots and solar flares, and these phenomena, in turn, cause space weather in Earth’s atmosphere, leading to beautiful visual effects (auroras) but troublesome disruptions of communications and electricity. Space weather also influences the Earth’s climate, weather, and seasons. All of this solar activity varies on a roughly eleven-year-long solar cycle. While these effects are mostly within Earth’s upper atmosphere, the Sun also directly affects life at the surface.

Sunlight drives photosynthesis in plants, algae, and some bacteria, allowing them to convert sunlight into food; without this process, the entire food chain would collapse. Humans also both enjoy and suffer direct effects from the Sun, which can damage human eyes and skin while also providing sanitizing ultraviolet rays and driving vitamin D production. Solar power can influence the future of technology. Earth’s ultimate demise will also be brought about by the Sun.

Effects of the Sun’s Magnetic Field and the Solar Cycle

The Sun releases energy in two main forms: electromagnetic radiation and charged particle emission. The activities of the Sun’s magnetic field and the different ways these types of energy are released and moved cause the Earth to experience various effects.

The Sun’s strong magnetic field has a complex spiral shape. The star’s high temperature means that it is composed solely of gases and plasma, allowing it to rotate faster around its equator than at the poles; this differential rotation twists the magnetic field into its distinctive form. Ultimately, the twists in the magnetic field lead to solar activity, such as sunspots and solar flares.

The magnetic field flips direction when solar activity is at its maximum in the eleven-year-long solar cycle. The Sun’s magnetic field exerts its influence well beyond the Sun itself because the solar wind, a plasma stream of charged particles, carries it through the solar system, altering the magnetic fields of the planets.

Varying solar activity, such as sporadic corona eruptions (coronal mass ejections), can alter the activity and intensity of the solar wind, resulting in geomagnetic storms on Earth. These storms cause beautiful glowing lights in Earth’s atmosphere called auroras, particularly aurora borealis (the northern lights) and aurora australis (the southern lights). On the negative side, though, the storms also interrupt communications on Earth, disrupting radio signals and electric transmissions and interfering with compass-based navigation, among other effects.

The frequency of geomagnetic storms roughly aligns with the sunspot cycle, which is determined by the solar cycle. Sunspots are visual representations of the Sun’s magnetic field; when the Sun’s magnetic field lines emerge from within the sun, they create magnetic loops in the photosphere (the Sun’s visible surface). The temperature is lower within these loops, appearing as dark patches called sunspots. Other solar activity includes solar flares and solar plumes.

The solar cycle, which usually takes eleven years, is marked by a solar maximum (at which time solar activity is at its highest level) and, conversely, a solar minimum. For example, during a solar maximum in 1859, a geomagnetic storm was so intense that the northern lights could be seen as far south as Rome. Several prolonged solar minimums have been observed throughout history, during which Earth temperatures were at an all-time low. One such period, known as the Maunder Minimum, spanned the second half of the seventeenth century and into the beginning of the eighteenth century.

The Earth-Sun relationship is partially driven by the solar cycle, during which solar activity always varyies. The Earth-Sun relationship is also strongly influenced by its orbit around the Sun and its rotation around its axis, which directly affects the distance between the Earth and the Sun. Perihelion (when the distance between Earth and the Sun is shortest) occurs once a year in early January, and aphelion (when the distance is longest) occurs once a year, in early July. These generally occur two weeks after the December solstice and June solstice, respectively.

The solstices are the two points in the year when the Sun appears highest in the sky, either in the north or in the south, because of the tilt of the Earth’s axis. The northern (or summer) solstice occurs in June, corresponding with summer in the Northern Hemisphere and winter in the Southern Hemisphere. Conversely, the southern (or winter) solstice occurs in December, during winter in the Northern Hemisphere and summer in the Southern Hemisphere. Similarly, two equinoxes occur each year, one in late March and one in late September, when the tilt of the Earth’s axis is such that the Sun is aligned in the same plane as the equator. The equinoxes are hallmarks of spring and autumn.

The big picture is that variations in the Sun’s magnetic field cause different types of space weather, which affect Earth’s upper atmosphere and ultimately influence Earth’s climate, weather, and seasons; the degree to which space weather occurs remains a topic of ongoing research. The ever-changing distance between the Earth and the Sun also influences the whole relationship.

The relationship between the Sun and the Earth was on brilliant display in May 2024 when the most significant solar storm in two decades produced large solar flares and coronal mass ejections. During this event, auroras were visible at unusually low latitudes. The National Aeronautics and Space Administration (NASA) was forced to take precautionary measures for missions and satellites. As global climate change continues to be a critical issue, scientists continue to study the relationship between sunspot activity and Earth’s climate. 

Photosynthesis and Health Effects of the Sun

The Sun’s effects on Earth are not limited to climate control; the Sun is also entirely responsible for the food chain (through photosynthesis), and it has several other health effects, both positive and negative.

Photosynthesis is a process that occurs in plants, algae, and some bacteria. In basic terms, the process involves sunlight reacting with carbon dioxide and water to yield sugar (food) and oxygen. To elaborate somewhat, the first stage of photosynthesis, light-dependency, begins with an organism trapping and storing sunlight. The sunlight’s energy is then converted into chemical energy, stored in the molecules ATP and NADPH.

The second stage does not require any more light; the already-created ATP and NADPH drive the reduction of carbon dioxide to glucose (sugar) and other useful organic molecules. Photosynthesis serves as the basis for the food chain and maintains the necessary oxygen levels in the atmosphere because oxygen is a waste product of the process.

The human health implications of sunlight, particularly the ultraviolet (UV) radiation in sunlight, are both positive and negative. On the positive side, human bodies synthesize vitamin D from sunlight; a deficiency in vitamin D can lead to the thinning and softening of bones, and some research has suggested a link between vitamin D deficiency and seasonal affective disorder, a seasonal depression.

On the negative side, too much UV radiation can lead to skin aging, skin cancer, and eye damage, such as cataracts and age-related macular degeneration. Looking at the Sun through binoculars without a proper UV filter can cause instant retinal damage and even permanent blindness. Looking at the Sun directly or with an optical aid during an eclipse is particularly dangerous.

Solar Power

Much as light energy can be converted to food by photosynthesis, it can also be converted to electricity. The harnessing of solar power began on a commercial scale in the 1970s and 1980s, and it continues to gain prevalence. It shows promise for the future because of its numerous advantages over other power sources. Most importantly, light energy is renewable: Sunlight will not be depleted, at least not while the Sun is still in roughly its current form, for the next several billion years.

Solar power is also a clean energy; it does not release greenhouse gases or pollute the ocean, as does oil. Additionally, solar cells are long-lasting and require little maintenance. Although renewable, sunlight is not always available. Storage solutions are essential because solar energy cannot be collected at night.

There are two major methods of solar power: first, the use of photovoltaics, which involves the use of the photoelectric effect of sunlight to convert light energy directly into an electric current, and second, the use of concentrated solar power, which involves the use of lenses or mirrors to focus sunlight into a small beam. This focused light is then converted to heat, which powers a steam turbine or other heat engine to generate electricity. Both technologies continue to evolve, and the hope is that costs will decrease and efficiency will increase in the coming decades.

The End of Earth

Just as the Sun has supported more than 4 billion years of life on Earth, it is ultimately the Sun that will bring about the end of Earth. The Sun is a main-sequence (or dwarf) star, marked by the fusion of hydrogen to helium within its core.

It is estimated that the Sun will exist for about 10 billion years in its main-sequence form before it has fused all of its hydrogen; it has already covered nearly half of that time span. This means that in about 5 billion years, it will enter its next phase of life as a red giant.

The outer layers of the red giant Sun will expand as its core contracts and heats up. This overall decrease in mass will likely loosen its gravitational pull on the solar system, allowing the planets to move outward. The Sun’s radius will be greater than the current distance between the Sun and the Earth, and it is unclear whether the looser pull will allow Earth’s orbit to move far enough away or if Earth will be swallowed up by the Sun. Even if the Earth does escape being enveloped by the Sun, the heat will still likely be enough to boil off all of Earth’s water and destroy much of its atmosphere. Even before any of this occurs, though (perhaps 1 billion years from now), life on Earth may cease to exist because of rising surface temperatures of the Sun (and thus the Earth).

Principal Terms

aphelion: the time at which the distance between the Earth and the Sun is smallest; generally occurs on one of the first days of January, two weeks after the December solstice

aurora: a glowing light display resulting from charged particles from solar wind being pulled into Earth’s atmosphere by Earth’s magnetic field; most often visible near Earth’s North and South Poles

equinox: a twice-a-year occurrence during which the tilt of the Earth’s axis is such that the Earth is not tilted toward or away from the Sun; the center of the Sun is directly aligned with Earth’s equator

geomagnetic storm: the effect of variations in solar wind’s interactions with Earth’s atmosphere; can result in communications disruptions and auroral displays in lower than usual latitudes

nuclear fusion: atomic nuclei join together to form a heavier nucleus; in the Sun and other main-sequence stars, hydrogen is fused to form helium

perihelion: the time at which the distance between the Earth and the Sun is largest; generally occurs on one of the first days of July, two weeks after the June solstice

solar cycle: an approximately eleven-year-long cycle of varying solar activity; solar cycles are tracked based on the visibility of sunspots

solar wind: a stream of charged particles that the Sun’s atmosphere ejects into space, where it can interact with the magnetic fields of planets

solstice: a twice-a-year occurrence during which the Sun appears at its highest point in the sky (once a year as seen from the North Pole and once a year as seen from the South Pole)

sunspot: a cooler area on the Sun’s surface that appears darker than the surrounding area; a zone of decreased temperature resulting from the complex shape of the Sun’s magnetic field

Bibliography

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