Solar Energy to Earth & the Seasons; Earth's Atmosphere PGS 45-58; 66-73

Solar System Formation

According to prevailing theory, our Solar System condensed from a large, slowly rotating and collapsing cloud of dust and gas known as a nebula. Gravity, the mutual attraction exerted by every object upon all other objects in proportion to their mass, was the key force in this condensing solar nebula. As the nebular cloud organized and flattened into a disk shape, the early protosun grew in mass at the center, drawing more matter to it. Smaller bodies of accreting material swirled at varying distances from the center of the solar nebula, becoming the protoplanets. The planetesimal hypothesis, or dust-cloud hypothesis, explains how suns condense from nebular clouds. In this hypothesis, small grains of cosmic dust and other solids accrete to form planetesimals that may grow into protoplanets and eventually planets in orbit around the developing Solar System's central mass.

Astronomers study this formation process in other parts of the galaxy, where planets are observed orbiting distant stars. By 2013, astronomers had discovered more than 4,400 candidate exoplanets orbiting other stars, with nearly 1,000 confirmed. Initial results from the orbiting Kepler telescope estimate that there are about 50 billion planets in the Milky Way, with some 500 million of them located in habitable zones—areas with moderate temperatures and liquid water—a staggering discovery. In our Solar System, around 165 moons (planetary satellites) are in orbit about six of the eight planets. As of 2012, the new satellite count for the four outer planets included Jupiter with 67 moons, Saturn with 62 moons, Uranus with 27 moons, and Neptune with 13 moons.

Dimensions and Distances

The speed of light is approximately 300,000 km/s, equivalent to about 9.5 trillion kilometers per year. This immense distance is captured by the term light-year, which serves as a unit of measurement for the vast universe. More precisely, the speed of light is about 299,792 km/s. For spatial comparison, our Moon is an average distance of 384,400 km from Earth, or about 1.28 seconds in terms of light speed; for the Apollo astronauts, this distance required a 3-day space voyage. Our entire Solar System spans roughly 11 hours in diameter, as measured by light speed. In contrast, the Milky Way galaxy is about 100,000 light-years wide, and the observable universe stretches approximately 12 billion light-years in all directions.

Earth's average distance from the Sun is about 150 million km, meaning light takes about 8 minutes and 20 seconds to reach Earth from the Sun. Earth's orbit around the Sun is currently elliptical, which is a closed, oval path. At perihelion, Earth's closest position to the Sun, occurring on January 3, the Earth-Sun distance is 147,255,000 km. At aphelion, Earth's farthest position from the Sun, occurring on July 4, the distance is 152,083,000 km. This seasonal variation in distance from the Sun causes a slight fluctuation in the solar energy received by Earth, although this is not the immediate cause of seasonal changes. The structure of Earth's orbit is not constant but undergoes long-term changes. Earth's distance from the Sun varies by more than 17.7 million km over a 100,000-year cycle, affecting the perihelion and aphelion distances at different times in the cycle.

Solar Energy: From Sun to Earth

The Sun, unique to us, is considered a typical star in the galaxy. It is average in temperature, size, and color when compared with other stars, yet it serves as the ultimate energy source for most life processes in our biosphere. The Sun captured about 99.9% of the matter from the original solar nebula. The remaining 0.1% formed all the planets, their satellites, asteroids, comets, and debris. Consequently, the dominant object in our region of space is the Sun, and it is the only object in the entire Solar System that possesses the enormous mass required to sustain nuclear reactions in its core and produce radiant energy.

The solar mass generates immense pressure and high temperatures deep within its dense interior. Under these conditions, the Sun's abundant hydrogen atoms fuse together, with pairs of hydrogen nuclei combining to form helium, the second-lightest element in nature, releasing vast amounts of energy in the process. The Sun's principal outputs include the solar wind and radiant energy that spans portions of the electromagnetic spectrum.

During its 4.6-billion-year existence, the Sun, Earth, and the other planets have completed 27 orbital trips around the Milky Way galaxy. When considering this travel distance alongside Earth's orbital speed around the Sun of 107,280 km/h and its equatorial rotation on its axis of 1,675 km/h, one begins to realize that the concept of being still is quite relative.

Solar Activity and Solar Wind

Telescopes and satellite images provide insights into solar activity, observed as sunspots and other surface disturbances. The solar cycle is the periodic variation in the Sun's activity and appearance over time. Since the advent of telescopes that allowed for sunspot observation in the 1800s, scientists have utilized these features to define the solar cycle. Observations have notably improved through data collected by satellites and spacecrafts, such as NASA's Solar Dynamics Observatory (SDO) and SOHO (Solar and Heliospheric Observatory). Sunspots are conspicuous features that appear dark on the solar surface and are caused by magnetic storms, varying in diameter from 10,000 to 50,000 km and some reaching as large as 160,000 km.

A solar minimum represents a period when few sunspots are visible, while a solar maximum is when sunspots are numerous. Over the past 300 years, sunspot occurrences have cycled in a fairly regular pattern, averaging 11 years from maximum to maximum. The previous solar cycle, Solar Cycle 24, commenced with a minimum in 2008 and reached a maximum in 2014. Scientists have dismissed solar cycles as a contributing factor to the increasing temperature trends on Earth observed over recent decades.

During solar maximum, solar flares, magnetic storms that cause surface explosions, and prominence eruptions—a form of gaseous outburst from the surface—often occur around sunspots. Not all material from these eruptions is pulled back toward the Sun by gravity; some moves into space, forming the solar wind. The Sun continuously emits clouds of electrically charged particles (mainly hydrogen nuclei and free electrons) that surge outward from the Sun's surface. Traveling slower than light, at approximately 50 million km per day, the solar wind takes around three days to reach Earth. This wind originates from the Sun’s extremely hot outer atmosphere, or corona, observable with the naked eye during a solar eclipse.

As the charged particles of the solar wind approach Earth, they interact with Earth's magnetic field, which surrounds Earth and extends beyond the atmosphere due to dynamo-like motions within our planet. The magnetosphere deflects the solar wind toward both of Earth's poles, allowing only a small fraction to enter the upper atmosphere. As the solar wind does not reach Earth's surface, research on this phenomenon is done in space. In 1969, Apollo 11 astronauts used a piece of foil on the lunar surface as a solar wind experiment, which when analyzed back on Earth confirmed the character of the solar wind. The charged solar wind particles and electromagnetic radiation from the Sun also pose risks to satellites, spacecraft, and electrical systems on Earth. Coronal mass ejections (CMEs) directed towards Earth can create spectacular auroras in the upper atmosphere near the poles, resulting in the aurora borealis (northern lights) and aurora australis (southern lights).

Electromagnetic Spectrum of Radiant Energy

The essential solar input to life is electromagnetic energy of various wavelengths, traveling at the speed of light to Earth. Solar radiation occupies a portion of the electromagnetic spectrum, which encompasses all wavelengths of electromagnetic energy. A physical law known as Wien's Displacement Law states that all objects radiate energy in wavelengths related to their surface temperatures. The Sun emits a composition of 8% ultraviolet, X-ray, and gamma-ray wavelengths; 47% visible light wavelengths; and 45% infrared wavelengths. The Sun’s surface temperature is approximately 6000 K (6273 °C), and it emits shorter wavelength energy, while Earth, being cooler, emits longer wavelengths predominantly in the infrared spectrum. Earth's curvature leads to variations in the angle at which sunlight strikes the surface, causing insolation to be more concentrated at lower latitudes, particularly between the tropics, which experience more direct sunlight throughout the year.

Reasons for Seasons

Earth's seasonal variations stem from the interplay of several factors: its revolution around the Sun, its axial tilt, and the unchanging orientation of its axis. These factors influence the Sun's altitude above the horizon and the duration of exposure to sunlight, referred to as daylength. Over the course of a year, Earth's axial tilt causes the Sun to appear higher or lower in the sky, which in turn affects warming and heating patterns across different regions. For instance, areas in the tropics experience relatively consistent daylength and temperatures year-round, while locations further from the equator, like mid-latitudes, see significant variations throughout the seasons. The seasonal shifts lead to distinct changes in weather patterns, vegetation, and overall ecological cycles, altering as they respond to shifting patterns of solar radiation and Earth's movement in its orbit around the Sun.

Understanding this dynamic interplay is essential in grasping the complexities of Earth's climate system, as global circulation influenced by solar energy continues to play a critical role in shaping weather, oceans, and life on Earth.