Page 81 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 2 solar energy to earth and the seasons 45
Solar System Formation
According to prevailing theory, our Solar System con- densed from a large, slowly rotating and collapsing cloud of dust and gas, a nebula. Gravity, the mutual at- traction 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 orga- nized and flattened into a disk shape, the early proto- sun grew in mass at the centre, drawing more matter to it. Small eddies of accreting material swirled at varying distances from the centre of the solar nebula; these were the protoplanets.
The planetesimal hypothesis, or dust-cloud hypoth- esis, explains how suns condense from nebular clouds. In this hypothesis, small grains of cosmic dust and other sol- ids accrete to form planetesimals that may grow to become protoplanets and eventually planets; these formed in or- bits about 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. In fact, by 2013, astronomers had discov- ered more than 4400 candidate exoplanets orbiting other stars, nearly 1000 confirmed. Initial results from the or- biting Kepler telescope estimate the number of planets in the Milky Way at 50 billion, with some 500 million plan- ets in habitable zones (with moderate temperatures and liquid water)—a staggering discovery.
Closer to home, in our Solar System, some 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, including several awaiting confirma- tion, was Jupiter, 67 moons; Saturn, 62 moons; Uranus, 27 moons; and Neptune, 13 moons.
Dimensions and Distances
The speed of light is 300000 km·s−1*—in other words, about 9.5 trillion km per year. This is the tremendous dis- tance captured by the term light-year, used as a unit of measurement for the vast Universe.
For spatial comparison, our Moon is an average dis- tance of 384400 km from Earth, or about 1.28 seconds in terms of light speed; for the Apollo astronauts this was a 3-day space voyage. Our entire Solar System is approxi- mately 11 hours in diameter, measured by light speed (Figure 2.1c). In contrast, the Milky Way is about 100000 light-years from side to side, and the known Universe that is observable from Earth stretches approximately
*In more precise numbers, speed of light is 299792 km·s−1.
12 billion light-years in all directions. (See a Solar Sys- tem simulator at space.jpl.nasa.gov/.)
Earth’s average distance from the Sun is approximately 150 million km, which means that light reaches Earth from the Sun in an average of 8 minutes and 20 seconds. Earth’s orbit around the Sun is presently elliptical— a closed, oval path (Figure 2.1d). At perihelion, which is Earth’s closest position to the Sun, occurring on January 3 during the Northern Hemisphere winter, the Earth–Sun distance is 147255000 km. At aphelion, which is Earth’s farthest position from the Sun, occurring on July 4 dur- ing the Northern Hemisphere summer, the distance is 152 083 000 km. This seasonal difference in distance from the Sun causes a slight variation in the solar energy in- coming to Earth but is not an immediate reason for sea- sonal change.
The structure of Earth’s orbit is not constant but in- stead changes over long periods. As shown in Chapter 11, Figure 11.16, Earth’s distance from the Sun varies more than 17.7 million km during a 100000-year cycle, causing the perihelion and aphelion to be closer or farther at dif- ferent times in the cycle.
Solar Energy: From Sun to Earth
Our Sun is unique to us but is a commonplace star in the Galaxy. It is average in temperature, size, and colour when compared with other stars, yet it is the ultimate en- ergy 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% of the matter formed all the planets, their satellites, asteroids, comets, and debris. Consequently, the dominant object in our re- gion of space is the Sun. In the entire Solar System, it is the only object having the enormous mass needed to sustain a nuclear reaction in its core and produce radiant energy.
The solar mass produces tremendous pressure and high temperatures deep in its dense interior. Under these conditions, the Sun’s abundant hydrogen atoms are forced together and pairs of hydrogen nuclei are joined in the process of fusion. In the fusion reaction, hydrogen nuclei form helium, the second-lightest element in na- ture, and enormous quantities of energy are liberated— literally, disappearing solar mass becomes energy.
A sunny day can seem so peaceful, certainly un- like the violence taking place on the Sun. The Sun’s principal outputs consist of the solar wind and of ra- diant energy spanning portions of the electromagnetic spectrum. Let us trace each of these emissions across space to Earth.
Georeport 2.1 Sun and Solar System on the Move
During the 4.6­billion­year existence of our solar system, the sun, earth, and other planets have completed 27 orbital trips around the milky Way galaxy. When you combine this travel distance with earth’s orbital­revolution speed about the sun of
107280 km·h−1 and earth’s equatorial rotation on its axis of 1675 km·h−1, you get an idea that “sitting still” is a relative term.