An Introduction to Astronomy

Objects in the Night Sky

• Our Solar System
• Stars
• Deep Sky Objects
• Closer to Home

How Do We Know?

Tours of the Night Sky

• Spring Tour
• Summer Tour
• Fall Tour
• Winter Tour

Glossary

Additional Reading, Resources, and Community Science

About the Author

Constellations: Mapping the Night Sky

The entire sky is too large to take in all at once. Since ancient times, people have identified patterns in the stars. Each pattern defines an area of the sky, which is called a constellation. Astronomers use constellations as a map of the sky. Every star in the sky is part of a constellation. The bright stars Betelgeuse and Rigel, for example, are in the constellation Orion. So is the famous star-forming nebula M42, which is also called the Orion Nebula.

What Can I See?

We can’t see constellation boundaries. They are made up. We see stars. So every constellation in the sky is drawn around a recognizable pattern of stars. The Sky Tours (page 107) in this book will help you find and recognize many of these patterns.

How Do We Know?

Constellations are made up. Throughout history, different cultures have recognized different constellations. Peoples all over the world, for example, recognized the shape of a person with broad shoulders and a narrow waist in the stars of the modern constellation we call Orion. The Ancient Greeks associated these stars with the hero Orion from their mythology. Many Hindu people associated them with Kaalpurush, a representation of their god Vishnu. To the Navaho these stars represented the protector Atse Ats’oosi, usually translated as “First Slender One.” They marked the movement of these stars to time the planting and harvest of crops.

Modern astronomy grew out of the ancient Greek tradition. The ancient Greeks, in turn, were influenced by the Babylonian, Sumerian, and Egyptian cultures of early history. Drawing on this long history, the International Astronomical Union recognizes 88 official constellations. These constellations are used by professional and most amateur astronomers worldwide to map the sky.

The Constellations of the Zodiac

The path of the sun in the sky is called the ecliptic. The area of the sky that surrounds the ecliptic is called the zodiac. The moon and planets always appear along the zodiac, near the ecliptic. The ecliptic passes through thirteen constellations. Twelve of these constellations share their names with the astrological “houses of the zodiac.”

What Can I See?

Look along the zodiac to spot the moon and planets. If you see a bright “star” near the ecliptic that doesn’t appear on a star chart, it is probably a planet.

How Do We Know?

People have always taken a special interest in the path of the sun through the sky. Ancient Greek astronomers divided the area around the ecliptic into twelve equal segments they called “houses” and named each after a nearby constellation. They called these houses the zodiac, which means “circle of animals.” Astronomers stopped using these houses centuries ago, but they continued using the constellations they were named after. Because of changes in the Earth’s spin over time, today’s constellations no longer line up with the ancient Greek zodiac. The dates the sun appears in each constellation are quite different from the traditional astrological dates. The chart on the next page shows the position of the sun throughout the year.

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