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Unit 04


Unit Overview

This unit presents a view of the Earth in the context of the universe, the many galaxies, and the solar system of which the Earth is a member. The main sections pertain to:

  • The universe
  • The solar system

The universe is an entity containing all the matter that exists anywhere in space and time. This entity is enormously large beyond imagination. In fact, distances are measured in light-years¿the distance traveled by a pulse of light in one year. The universe is comprised of galaxies, one of which is our galaxy¿the Milky Way. Within the Milky Way is our solar system.

The Sun is by far the most massive body in our solar system, which is also comprised of planets and residual materials. The planets revolve around the Sun, with orbit lengths (distance traveled) and periods (time taken to complete a revolution) differing depending on size, weight, and location. Just as the Sun has planets most of the planets also have moons. The Earth's Moon was created as the result of the collision of an object with the Earth. The Moon's many craters were created as objects in space have made contact with the Moon's surface. Unlike the Earth, the Moon does not have an atmosphere to act as a protective shield against collisions.

Unit Objectives

  • To introduce the basic structures of the universe, speculations about its origin, and the position of our home galaxy and star within it
  • To describe the functions of the Sun as the dominant body of the solar system
  • To briefly survey each of the Sun's nine planets and the lesser orbiting bodies that constitute the remainder of the solar system.

Glossary of Key Terms

Galaxy An organized, disk-like assemblage of billions of stars; our solar system belongs to the Milky Way galaxy, which measures about 120,000 light-years in diameter.
Gravity The force of attraction that acts among all physical objects as a result of the mass ( the quantity of material they are composed of).
Light-year The distance traveled by a pulse of light in one year. Light travels at a speed of 300,000 km (186,000 mi) per second; a light-year thus involves a distance of 9.46 trillion km (5.88 trillion mi).
Moon Satellite that orbits a planet, probably originating from the clustering of planetesimals. All planets except Mercury and Venus have such bodies; our Moon orbits the Earth once every 27.3 days at an average distance of 385,000 km (240,000 mi).
Planet A dark solid body, much smaller in size than a star, whose movements in space are controlled by the gravitational effects of a nearby star.
Revolution One complete circling of the Sun by a planet; it takes Earth precisely one year to complete such an orbit.
Rotation The spinning of a planet on its axis, the imaginary line passing through its center and both poles; it takes Earth one calendar day to complete one full rotation.
Solar system The Sun and its nine orbiting planets (plus their orbiting satellites). In order of increasing distance from the Sun, these planets are Mercury, Venus, Earth, Mars. Jupiter, Saturn, Uranus, Neptune, and Pluto.
Universe The entity containing all of the matter and energy that exists anywhere in space and time.

Unit Outline

  • The universe
    • A light-year measures distances and is about 9.46 trillion km or 5.88 trillion mi
    • Galaxies
      • Milky Way
      • local group
      • local supercluster
    • Big Bang Theory
      • explosion expelling all primordial energy and matter
      • gravity is the force of attraction among all objects due to their mass
  • The solar system
    • The Sun
      • solar wind
      • sunspots
    • The planets
      • revolution
      • the inner planets
        • Mercury
        • Venus
        • Earth
        • Mars
      • the outer planets
        • Jupiter
        • Saturn
        • Uranus
        • Neptune
    • The lesser bodies of the solar system
      • Pluto
      • moons
    • Smaller objects of the solar system
      • asteroids
      • comets
      • meteoroids (and meteorites)
      • dust

Review Questions

  1. Compare and contrast the relative sizes and distances from the Sun of the nine planets of our solar system.
  2. List the critical variables that make Earth a habitable environment - use Table 4.1 as a guide.
  3. Define the term sunspots, and describe some of their possible effects on Earth.