This section gives an overview of our universe. An introduction to astronomy and terms of measurement used in astronomy are also provided. The introduction to the origins of the universe covers the Big bang theory, Continuous creation theory and Pulsating (Oscillating) Universe theory. An overview of galaxies and constellations is also provided in this section.

Section 1: Objectives

After completing this section, the student will be able to:

1. summarize the evolution of astronomy.
2. describe the major theories about the origins of the universe.
3. explain the different types of galaxies and constellations.

Topic 1.1  Introduction

Men have always looked up at the sky and asked questions about what they saw. What makes the sun shine? What makes it hot? What are the stars, the moon, and the planets?

Meaning of Astronomy

Astronomy is the study of heavenly bodies, all the objects in the sky. It is the oldest science.

Astronomers try to answer these questions.

The word “Astronaut” comes from the Greek words astron (“star”) and nautes (“sailor”).

One task of astronomy is to keep track of all the heavenly bodies. At what time will the sun rise? At what time will it set? When will winter begin? When will it be warm enough to plant crops? These questions were very important to the first astronomers. They are still important to farmers, sailors, and other people.

Ancient astronomers studied the objects in the sky for a very long time. They finally began to notice patterns – events that happened again and again. One important pattern they saw was the phases of the moon.

Astronomers invented the first calendar using this pattern. Early calendars were not as accurate as the calendars used today. But they helped farmers and sailors, just as today’s calendars help people.

Astronomers in the ancient world could measure many things that happened in the sky. But they could not explain why these things happened.

The second main task of astronomy is to explain the causes of events in the sky.

The ancient Chinese built a big observatory 2,600 years before Christ. They could predict when eclipses would occur, but they did not know how to explain what happened. As a result, they were terrified of such happenings.

Greek Astronomy

The most difficult challenge was to explain how and why heavenly bodies seem to circle the Earth. Pythagoras, in 500 B.C. taught that the planets revolved around the sun. But not many people believed him. Plato and Aristotle, two important philosophers and astronomers, taught that the Earth is the center of the universe. They thought the sun, moon, stars, and planets were attached to giant spheres rotating around the Earth. This seemed easier to believe.

Claudius Ptolemy, a Greek who lived in Egypt around 100 to 170 A.D., wrote 13 books that “proved” that the Earth was the center of the universe. For centuriesmany people believed that he was right.

Nicolaus Copernicus, a sixteenth–century Polish astronomer, claimed that all the planets revolved around the sun. Again, people said he was wrong.

Johannes Kepler, a German scientist, used scientific measurements to show that the Earth and all other planets orbit (circle) the sun. Galileo, an Italian, described this fact after he began using a telescope – the tool that finally helped astronomers understand the movements of heavenly bodies.

Modern Astronomy

Galileo was the first astronomer to use a telescope. He discovered four moons of Jupiter, in 1610. He studied Venus and saw that the planet has phases, like the phases of the moon. Because the moon goes around the Earth, Venus, too, must revolve around something, obviously the sun. So, Galileo proved that the Earth is not the center of the universe.

Terms Used for Units of Measurement

No one knew the size of the universe in ancient times. Ptolemy knew that stars are farther away from the Earth than the planets are. But, if you could talk to Ptolemy and tell him how far away the stars really are, he probably not wouldn't believe you!

Astronomers have had to change their minds about how big the universe is 4 times in the last 50 years. Each time, scientists thought they knew the size of the universe an astronomer would find a new object farther away, and a new, even bigger estimate would be made.

If you said that New York and San Franciso are about 190,080,000 inches apart, you would be right. But most people would be confused, because this number is too large to comprehend. This is one reason people use miles as a unit of measure. People usually say the two cities are about 3,000 miles apart.

Astronomers have a similar problem with comprehension of units of measurement. Distances in space are so long that even miles do not indicate how faraway the objects in space are. So astronomers use other measuring units. Let’s have a look at them.

Astronomical Unit

To measure distances inside the solar system – the sun and its nine planets – astronomers use the astronomical unit (a. u). One a. u is the mean distance from the Earth to the sun, about 93 million miles. Astronomers say Pluto is about 39 a. u. from the sun. This is easier than saying three billion six hundred and twenty–seven million miles.

Light Years

Even an astronomical unit is too small a unit to measure the distance between the solar system to other parts of the universe. Astronomers measure these great distances in light years. In other words – the distance that light travels in a year. Light travels about 186,000 miles every second. In a year, that adds up to 5,880,000,000,000 miles! Light from the stars travels a long time to reach the Earth. The nearest star to Earth is Alpha Centauri. Its light takes about four years and four months to traverse the huge distance to Earth. Even light from the sun takes about eight minutes to make the journey to Earth.

In terms of space dimensions, we may say that a light year is made up of about 60,000 A.U.s.

Astronomers can now “see” about 12 ½ billion light years away from Earth, using telescopes and special radios. This means that the universe is at least 25 billion light years wide. (If you want to know how many miles that is, multiply 5.88 trillion times by 25 billions). No wonder Ptolemy wouldn’t believe you!

Topic Practice

Online Quiz
Students: You can take this online quiz as many times as you need in order to insure you understand the topic.