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15.1: Star Constellations

Difficulty Level: At Grade Created by: CK-12
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How did astrology come to be?

Ancient Babylonian astronomers created the Zodiac, a circle that divides the ecliptic into twelve 30-degree zones. Each zone contains a constellation, many of them animals. Horoscopes based on these astrological signs first appeared in Ptolemaic Egypt in around 50 BC. These early people used astrology to explain things that are now much better explained by science.


When you look at the sky on a clear night, you can see dozens, perhaps even hundreds, of tiny points of light. Almost every one of these points of light is a star, a giant ball of glowing gas at a very, very high temperature. Stars differ in size, temperature, and age, but they all appear to be made up of the same elements and to behave according to the same principles.


People of many different cultures, including the Greeks, identified patterns of stars in the sky. We call these patterns constellations. Figure below shows one of the most easily recognized constellations.

In this image the Big Dipper is outlined and shown next to the Aurora borealis near Fairbanks, Alaska.

Why do the patterns in constellations and in groups or clusters of stars, called asterisms, stay the same night after night? Although the stars move across the sky, they stay in the same patterns. This is because the apparent nightly motion of the stars is actually caused by the rotation of Earth on its axis. The patterns also shift in the sky with the seasons as Earth revolves around the Sun. As a result, people in a particular location can see different constellations in the winter than in the summer. For example, in the Northern Hemisphere Orion is a prominent constellation in the winter sky, but not in the summer sky. This is the annual traverse of the constellations.

Apparent Versus Real Distances

Although the stars in a constellation appear close together as we see them in our night sky, they are not at all close together out in space. In the constellation Orion, the stars visible to the naked eye are at distances ranging from just 26 light-years (which is relatively close to Earth) to several thousand light-years away.


How parallax works.


There is no reason to think that the alignment of the stars has anything to do with events that happen on Earth. The constellations were defined by people who noticed that patterns could be made from stars, but the patterns do not reflect any characteristics of the stars themselves. When scientific tests are done to provide evidence in support of astrological ideas, the tests fail. When a scientific idea fails, it is abandoned or modified. Astrologers do not change or abandon their ideas.


  • The points of light in the night sky are stars that are balls of gas undergoing nuclear fusion.
  • Constellations are patterns of stars that are usually not near each other but are the result of chance.
  • Stars in a constellation may be fairly close together, but are more likely extremely far apart.


Use these resources to answer the questions that follow.


1. What are constellations?

2. How many constellations are there?


3. What are the two groups of constellations? Define each.


4. List the constellations that are always visible in the Northern Hemisphere.

5. Why is Ursa Major unique?


1. Why are constellations so important to people when they think about stars?

2. Are constellations useful?

3. What is astrology? How is it different from astronomy?

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asterism A group or cluster of stars that appear close together in the sky.
constellation A group of stars that appear to form a pattern in the sky. Most often these stars are unrelated and are not near each other in space. Constellations are used to locate objects in space.
star A glowing sphere of gases that produces light through nuclear fusion reactions.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 07, 2016
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