I. The Sun-Earth-Moon System and The Moon: Earth is one of nine planets in the solar system. Earth is the third planet from the Sun and is located between the Sun and the asteroid belt. It has one natural satellite, the Moon. The Earth is one of four inner terrestrial planets. Unlike other inner planets, the Earth has large amounts of life-supporting water. Unique to Earth, water occurs as a solid, a liquid, or a gas. The Earth also has an oxygen-rich atmosphere. The daily apparent motions of the Sun and the Moon result from Earth’s rotation. Earth completes one revolution around the Sun every 365.25 days. The Earth’s revolution around the Sun, tilted on its axis, causes the seasons (equinoxes and solstices). The Sun’s rays are vertical at the Tropic of Cancer during the summer solstice, at the Tropic of Capricorn during the winter solstice, and at the equator during the autumnal and the vernal equinox.

The Earth’s moon is a rocky satellite that is about one-quarter the diameter of the Earth and one-eighth the mass of the Earth. It is highly cratered and it lacks an atmosphere. Therefore, there is no erosion to eliminate traces of impacts from space objects. The Moon revolves around the Earth once a month causing the moon phases and eclipses. Only one side of the moon is visible to Earth as its rotation is synchronous with the Earth. The phases of the Moon are caused by its position relative to the Earth and the Sun. The phases of the Moon include the new, crescent, gibbous, quarter, and full moon. Solar eclipses occur when the Moon blocks out sunlight from Earth’s surface. Lunar eclipses occur when Earth blocks sunlight from reaching the Moon’s surface. Tides are the daily, periodic rise and fall of water level caused by the gravitational pull of the Sun and the Moon (spring and neap).

II. Characteristics of Our Solar System: The solar system consists of many types of celestial bodies. There are essentially two types of planets in our solar system: rocky (terrestrial) planets and the gas giants. The four inner (terrestrial) planets consist mostly of solid rock. Of the terrestrial planets, Earth is the most dense. Four of the five outer planets (Jupiter, Saturn, Uranus, and Neptune) are gas giants consisting of thick, outer layers of gaseous materials (perhaps with small rocky cores). The fifth outer planet (Pluto) has an unknown composition, but it appears to be solid. Moons are natural satellites of planets that vary widely in composition. Comets orbit the sun and consist mostly of frozen gas. Asteroids are rocky or metallic iron objects ranging in size from millimeters to kilometers. They are the source of most meteorites. Kepler’s Laws govern planetary motion. Models of the solar system have evolved from geocentric to heliocentric.

III. The Sun and the Stars: The Sun consists largely of hydrogen gas. The Sun’s energy comes from the nuclear fusion of hydrogen to helium. The Sun’s atmosphere consists of the photosphere, the chromosphere, and the corona. Sunspots, solar flares, and prominences are active features of the Sun. Stars form whenever dense clouds of interstellar gas and dust (called nebulae) exist and collapse. Stars have a finite lifetime and evolve over time. The mass of a star controls its evolution, length of its lifetime, and ultimate fate. A star changes as it ages because its internal composition changes as nuclear fusion reactions in its core convert one element to another. The H-R diagram illustrates the relationship between the absolute magnitude and the surface temperature of stars. As stars evolve, their position moves on the H-R diagram. The main characteristics used to classify stars are size, temperature, and brightness. Red giants, blue giants, white dwarfs, and yellow dwarfs can be plotted on the H-R diagram according to their size and temperature.

IV. The Milky Way and Other Galaxies and Origin and Evolution of the Universe: Galaxies are collections of billions of stars. The solar system is located in the Milky Way galaxy. The basic types of galaxies are spiral, elliptical, and irregular. The Milky Way is a spiral galaxy. Cosmology is the study of the origin and processes for formation of the universe. The solar nebular theory is our best current hypothesis for the origin of the solar system. There are different theories that explain the creation of the universe including the oscillating universe theory and the Big Bang theory. The Big Bang theory is our best current model for the origin of the universe. Astronomers use a unit called a light-year to measure distance in space. A light-year is the distance the light travels in one year, or about 9.5 x 1012 kilometers. Much of our information about our galaxy and the universe comes from ground-based observations.

V. The Space Program: Much of our knowledge about the solar system is the result of space exploration efforts. There have been many major events in space exploration from manned missions to unmanned robotic probes. Apollo 11 was the first manned landing on the Moon. Space technology impacts our everyday lives by spurring the development of weather forecasting, satellite communications, the search for alternative energy sources, and medical advances. The Hubble Space Telescope provides views of the universe that cannot be made using ground-based telescopes or other satellites.