Earth's Place in the Universe

Earth's Place in the Universe opens by examining the astronomical observations and contributions made by early civilizations and the Greek philosophers, including Aristotle, Aristarchus, Hipparchus, as well as Ptolemy. An in-depth examination of the birth of modern astronomy centers on the contributions of Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, and Sir Isaac Newton. Following a brief overview of the constellations, a system for locating stars in the sky is presented. The primary motions of Earth are also described in detail. The chapter concludes with discussions of the phases of the moon, lunar motions, and eclipses.

Learning Objectives

After reading, studying, and discussing this chapter, you should be able to:

•Describe the geocentric theory of the universe held by many early Greeks.

•List the astronomical contributions of the ancient Greek philosophers Aristotle, Anaxagoras,

Aristarchus, Eratosthenes, Hipparchus, and Ptolemy.

•Describe the Ptolemaic model of the universe.

•List the contributions to modern astronomy of Nicolaus Copernicus, Tycho Brahe,

Johannes Kepler, Galileo Galilei, and Sir Isaac Newton.

•Describe the equatorial system for locating stars.

•List and describe the primary motions of Earth.

•Discuss the phases of the moon, lunar motions, and eclipses.

Chapter Summary

•Early Greeks held the geocentric ("Earth-centered") view of the universe, believing that Earth was a sphere that stayed motionless at the center of the universe. Orbiting Earth were the seven wanderers (planetai in Greek) which included the moon, sun, and the known planets-Mercury, Venus, Mars, Jupiter, and Saturn. To the early Greeks, the stars traveled daily around Earth on a transparent, hollow sphere called the celestial sphere. In A.D. 141, Claudius Ptolemy presented the geocentric outlook of the Greeks in its most sophisticated form in a model which became known as the Ptolemaic system. The Ptolemaic model had the planets moving in circular orbits around a motionless Earth. To explain the retrograde motion of planets (the apparent westward, or opposite motion planets exhibit for a period of time as Earth overtakes and passes them) Ptolemy proposed that the planets orbited in small circles (epicycles), revolving along large circles (deferents).

•In the fifth century 8.C., the Greek Anaxagoras reasoned that the moon shines by reflected sunlight, and because it is a sphere, only half is illuminated at one time. Aristotle (384-322 BC) concluded that Earth is spherical. The first Greek to profess a sun-centered, or heliocentric, universe was Aristarchus (312-230 B.C.). The first successful attempt to establish the size of Earth is credited to Eratosthenes (276-194 B.C.). The greatest of the early Greek astronomers was Hipparchus (second century B.C.), best known for his star catalog.

•Modern astronomy evolved through the work of many dedicated individuals during the 1500s and 1600s. Nicolaus Copernicus (1473-1543) reconstructed the solar system with the sun at the center and the planets orbiting around it, but erroneously continued to use circles to represent the orbits of planets. Tycho Brahe’s (1545-1607) observations were far more precise than any made previously and are his legacy to astronomy. Johannes Kepler (1571-1630) ushered in the new astronomy with his three laws of planetary motion. After constructing his own telescope, Galileo Galilei (1554-1542) made many important discoveries that supported the Copernican view of a sun-centered solar system. Sir Isaac Newton (1643-1727) was the first to formulate and test the law of universal gravitation, develop the laws of motion, and prove that the force of gravity, combined with the tendency of an object to move in a straight line (inertia), results in the elliptical orbits discovered by Kepler.

•As early as 5000 years ago people began naming the configurations of stars, called constellations, in honor of mythological characters or great heroes. Today, 88 constellations are recognized that divide the sky into units, just as state boundaries divide the United States.

•One method for locating stars, called the equatorial system, divides the celestial sphere into a coordinate system similar to the latitude-longitude system used for locations on Earth's surface. Declination, like latitude, is the angular distance north or south of the celestial equator. Right ascension is the angular distance measured eastward from the position of the vernal equinox (the point in the sky where the sun crosses the celestial equator at the onset of spring).

•The two primary motions of Earth are rotation (the turning, or spinning, of a body on its axis) and revolution (the motion of a body, such as a planet or moon, along a path around some point in space), Another very slow motion of Earth is precession (the slow motion of Earth's axis which traces out a cone over a period of 26,000 years). Earth's rotation can be measured in two ways, making two kinds of days. The mean solar day is the time interval from one noon to the next, which averages about 24 hours on the other hand, the sidereal day is the time it takes for Earth to make one complete rotation with respect to a star other than the sun, a period of 23 hours, 56 minutes, and 4 seconds. Earth revolves around the sun in an elliptical orbit at an average distance from the sun of 150 million kilometers (93 million miles). At perihelion (closest to the sun), which occurs in January, Earth is 147 million kilometers from the sun. At aphelion (farthest from the sun), which occurs in July, Earth is 152 million kilometers distant. The imaginary plane that connects Earth's orbit with the celestial sphere is called the plane of the ecliptic.

•One of the first astronomical phenomenon to be understood was the regular cycle of the phases of the moon. The cycle of the moon through its phases requires 29½ days, a time span called tie synodic month. However, the true period of the moon's revolution around Earth takes 27 1/3 days and is known as the sidereal month. The difference of nearly two days is due to the fact that as the moon orbits Earth, the Earth-moon system also moves in an orbit around the sun.

•In addition to understanding the moon's phases, the early Greeks also realized that eclipses are simply shadow effects. When the moon moves in a line directly between Earth and the sun, which can occur only during the new-moon phase, it casts a dark shadow on Earth, producing a solar eclipse. A lunar eclipse takes place when the moon moves within the shadow of Earth during the full-moon phase. Because the moon's orbit is inclined about 5 degrees to the plane that contains the Earth and sun (the plane of the ecliptic), during most new- and full-moon phases no eclipse occurs. Only if a new- or full-moon phase occurs as the moon crosses the plane of the ecliptic can an eclipse take place. The usual number of eclipses is four per year.