The sun gives a rhythm to our lives – day follows night, shadows grow shorter and longer, the seasons follow a regular progression over the year. We are adapted to these daily and seasonal changes, and our lives are structured around them. Bats hunt at night and dragonflies in the daytime, leaves turn colors in the fall, buds swell in the spring. Whether plant or animal, diurnal or nocturnal, all living things depend on the sun for providing heat and light and making our planet habitable.
At the hub of our solar system, the sun is enormous compared to its planets, measuring some 860,000 miles in diameter – so big that a million Earths could fit inside. Because it contains over ninety-nine percent of the mass in our solar system, it has a huge gravitational pull, holding the planets in orbit. Like all other stars, the sun is made of glowing plasma. Plasma is a fourth state of matter so full of energy that atoms are ionized, with electrons separated from their atoms and from each other. Plasma is uncommon on Earth, but artificial plasmas are used in fluorescent lights, arc welders, and plasma televisions. Energy is generated inside the sun by nuclear fusion – the combining of hydrogen atoms to form helium. In the process, energy is released as electromagnetic radiation which includes visible light and heat (infrared radiation), as well as gamma rays, x-rays, radio waves, and others. Although we are a small planet, ninety-three million miles away from the sun, the tiny fraction (one one-billionth) of this radiation that reaches our planet provides sufficient energy for life to exist.
Energy from the sun keeps our planet at temperatures in which living things can survive. Though scientists are finding life forms that can exist in extreme habitats, exposed to intense heat, cold, dryness, or pressure, most living creatures on Earth thrive in a much narrower range of conditions. Insects, spiders, and other invertebrates, as well as fish, reptiles, and amphibians, don’t generate their own body heat (we call them “cold-blooded”) and need the sun’s warmth to function. Even birds and mammals, warm-blooded animals that produce heat by metabolizing food, cannot survive very cold temperatures.
Sunlight also powers the water cycle, the movement of water between the Earth and its atmosphere. Heated by the sun, water constantly evaporates from oceans, lakes, and streams. Once in the air, water vapor condenses into clouds that eventually produce rain or snow, returning water to the oceans and replenishing the supply of freshwater needed by terrestrial life.
Sunlight provides the energy that plants and animals need to live and grow. Plants are able to harness the sun’s energy through the process of photosynthesis – their leaves absorb sunlight and use the energy to build leaves, stems, fruits and seeds. Animals, unable to convert sunlight into food, depend on plants for nourishment or they eat other animals that eat the plants. So plants form the foundation of the food chain on Earth, and without sunlight plants and animals would not be able to exist here.
Every day the sun appears to rise in the east, move in an arc across the sky, and sink beneath the horizon in the west. Though it might seem as though the sun is traveling around the Earth, in fact, the Earth is rotating on its axis, spinning in a counterclockwise direction (when viewed from above the North Pole), and making one full turn every twenty-four hours. When it’s daytime, our side is facing the sun, and at night we are on the side away from the sun.
The angle of sunlight reaching the Earth has important effects on climate, ecology, and seasonal variations. Direct rays of light (reaching a surface at a right angle) are hotter because they deliver the same amount of energy to a smaller area than slanted rays do. This can be easily seen if you point a flashlight straight at a surface or from an angle. The angled light delivers the same energy but it is spread out over a greater area and so is dimmer. Because the Earth is a sphere, the sun’s rays are more direct over the tropics and less direct at the poles. It is coldest at the poles because here the sun’s light is most slanted and weakest.
How do we explain the seasons and changes in the length of the day? Besides rotating on its axis, the Earth revolves around the sun, taking one year to make a complete orbit. The Earth’s axis is tilted about 23.5 degrees from vertical, and this tilt stays the same as we travel around the sun. When the northern hemisphere is tilted towards the sun, we experience summer because the direct rays of the sun are above the equator, centered over the Tropic of Cancer. We experience winter at the other end of our orbit, when we are tilted away from the sun and the direct rays are over the Tropic of Capricorn, below the equator. At the spring and fall equinoxes, the tilt is parallel to the sun, the direct rays centered on the equator, so we experience equal days and nights and seasons that are intermediate between summer and winter.
We can see through something if it allows light to pass through. Light waves can pass through materials that are transparent, like glass, water, and some plastics. Materials like frosted glass, plastic milk bottles, and leaves are translucent, allowing only some light to pass through. We can see light but not objects through them. Other materials, such as metals, wood, many rocks, and some dark-colored plastics, are completely opaque, blocking all light. In general, the thicker the material, the harder it is for light to pass through, unless it is very clear, like black ice on ponds in winter.
If we shine a light on an opaque object that is smaller than the light source, we see a shadow behind it. This is because light waves travel in straight lines and do not curve or spread out. Trees, bushes, rocks, and hills cast shadows that create places where the temperature is lower and humidity higher than in direct sunlight. We find ferns and mosses on damp and shady forest floors, trout in shaded streams. Shadows change over the course of the day, being longest at dawn and dusk and shortest at midday when the sun is overhead. In the morning, with sunlight coming from the east, shadows are on the west sides of trees and fence posts, and in the afternoon, they gradually stretch out to the east. Shade and shadows offer us relief from the heat of the sun and provide clues about the time of day.
Solar panels capture sunlight directly and turn it into electricity or thermal energy. Passive solar design, as in greenhouses, works by taking advantage of the way light energy is transformed into heat. Light rays have short wavelengths and can pass through glass or transparent plastic (as well as air and water). They are absorbed by dark surfaces inside that then radiate heat waves, which, because they have longer wavelengths, can’t escape through the glass. Clouds in our atmosphere behave like the glass of a greenhouse, letting in light but then trapping heat radiated by the Earth, which keeps our planet warm. Increasing levels of carbon dioxide and water vapor in the air due to man’s activities is causing the Earth’s climate to warm.
Humans use fuels to power our machines and heat our homes, and these ultimately come from the sun as well. Hydroelectric power comes from flowing rivers, which are a part of the water cycle, driven by the sun. Fossil fuels like coal, oil, and natural gas were formed from the remains of ancient plants and animals, and firewood comes from present-day trees. Unequal heating of the Earth by the sun creates air currents – the wind that supplies us with wind power.
Sunlight warms the Earth, fuels the water cycle, and provides the foundation for our food chains, while shadows and shade create a variety of conditions where different plants and animals can live. As the Earth spins on its axis and travels around the sun, we experience daily and monthly changes that set the pace of our lives.
Golub, Leon, and Jay M. Pasachoff, Nearest Star: The Surprising Science of Our Sun, Harvard University Press, 2002.
Hamblin, W. Kenneth and Eric H. Christiansen. Earth’s Dynamic Systems. 10th ed. New Jersey: Prentice-Hall, 2004.
Lehr, Paul E., R. Will Burnett, and Herbert S. Zim. Weather. A Golden Guide. New York: St. Martin’s Press, 2001.