Charting the Heavens

Early people thought the Earth was the center of the Universe. Nowadays, we know we live on a planet that orbits a star, and together we orbit the center of our galaxy. Our galaxy is one of many billions of galaxies in the universe.


In this section, we will learn about our place in the universe, as well as properties of the Earth that pertain to the Sun-Earth-Moon system, such as phases of the moon, eclipses and seasons.

constellations in the night sky


  • Scientific method
  • Critical thinking
  • Earth's orbital motion
    • Seasons
    • Solar day vs. Sidereal day
  • Sun-Earth-Moon system
    • Phases of the moon
    • Eclipses
  • Measuring distance
    • Parallax
  • Viewing the universe
    • Constellations
    • The Celestial Sphere

The Scientific method

In the early days of astronomy, people thought that the Earth was the center of the universe.


It was believed that the sun, moon and stars orbited Earth. This is understandable, since it looks that way to us. We even still say that the sun rises in the morning, even though we now know that the apparent motion of the sun through the day is due to the rotation of the Earth on its axis.


The rise of scientific knowledge has come about because of the tireless, painstaking work of very many people, adhering to the scientific method. Although there is not really one form of the scientific method, it is generally recognized that this method entails the connection between observation, theory and experimental verification.

A theory is a group of related ideas that depend on a set of assumptions. A theory can form a framework that is used to make predictions. These predictions need to be based on proven facts. The predictions are then applied to observations. Upon making observations, a theory may be revised or extended to include more details to make new predictions.


Notice that we are using the word "theory" a little differently than it can be used in everyday conversation. In the scientific sense, a theory contains many facts as a basis. It is not just conjecture.


The Scientific Method is used to further science based on known facts. Often, it is used in the context of laboratory experiments. A lab experiment needs to be well-documented and reproducible.


In the case of astronomy, experiments cannot be done in the same way that they are done in other fields, like chemistry for example. In a good experiment, a scientists holds all variables fixed except for one, and manipulates just that one aspect to see what effect is has on the system. We cannot do the same thing with stars and planets. We can't go to a star and change some aspect of the star and then measure the result. What we can do is gather light from the star and make measurements of the light. We can also build models of stars on computers, using the basic laws of physics. Agreement between computational modeling and observation provides us with an increasingly richer understanding of how stars, planets and the universe work.


Another issue with observational astronomy is that the time frames for significant evolution in astrophysical systems can be extremely long, compared to a human lifetime. How can we hope to understand processes that take millions of years to develop?


One way to answer this question would be to consider what you might think about human beings if you were only able to observe the Earth for a short time, very much shorter than a human lifespan. The fact that there are many people living on the Earth would give you a lot of information, even in a short time. You would see people of different sizes and ages, and could possibly draw conclusions about how people change as they age.


Similarly, we have only a short snapshot of the stars, but there are very many stars populating our galaxy. We can infer much about the evolution of stars from our observations of large numbers of stars. As for planets, our technology has only recently advanced to the point where we can view extrasolar planets, outside our own solar system. As we learn more about other solar systems, our theories about our own solar system formation must evolve to include observations of solar systems in general.

We no longer believe that Earth is the center of the universe. We now know that Earth is a planet that orbits a star, one of the roughly 200 billion stars in our galaxy. Our galaxy is one of the roughly 200 billion galaxies in our known universe. We know this because of long, time exposure photographs taken by the Hubble telescope, called deep-field images. To take images like these, the Hubble telescope was trained on a relatively empty-looking area of the night sky, taking successive observations. Please visit to learn more.


The Hubble Deep field image shows us that there are very many galaxies, some similar to our own Milky Way galaxy. Since we reside within the Milky Way, we cannot yet see how it appears from a distance. Perhaps some day we will venture outside our galaxy, but for now, our earliest probes are barely leaving our solar system. Even so, astronomers have done a lot of work to understand our galaxy and how it is laid out in space. Estimates vary between 100 billion and 200 billion as to how many stars are present in the Milky Way.


The Milky Way is some 100,000 light years across. It is extremely hard for us to grasp distances that large. One way to gain an understanding of this vast distance is to compare it to sizes in our everyday lives, in decreasing scales by powers of ten. The link below takes you to one such journey, beginning with a representation of the Milky Way galaxy, decreasing to our everyday world, and continuing on down to subatomic sizes.

Using the scientific method and thinking about things in a scientific way involves thinking critically about each idea as is is addressed and meshed in with other ideas. For example, how do you know if global warming is real or not? An internet search will provide you with contrasting opinions about it. You need to learn how to evaluate varying sources with an objective eye and recognize which arguments are based on scientific fact.


Learning how to think critically is one of the foundational reasons for higher education.


To begin to understand critical thinking, we need to examine what learning is.


Critical thinking

This diagram illustrates the levels of learning known as Bloom's Taxonomy.


The very lowest level of learning is remembering. This is simply being told something and being able to repeat it. This "telling" can be verbal, written, images, or any other way of inputting information.


The next higher level is understanding what you have been told. This involves putting ideas together to make a higher contextual sense.


Applying that understanding is the hallmark of the next higher level. This involves being able to take the information and process it to use the concept in a different situation.


Analysis is the next higher level of learning. This is the capability of breaking down the information into elements that can be related to each other, possibly in multiple ways.


Evaluation is judging that analysis to make decisions based on the relevance of the ideas and their relationships with each other.


Creativity is the highest level of learning. It is the ability to take what you have learned, process it using all of the other aspects of learning, and make something new.


Higher level learning is the hallmark of scientific thought. One of the basic reasons for a college education is to instigate higher levels of learning. This is especially important in a science class. You need to be able to not only remember what you have seen, but to understand and apply the concepts to situations that are similar in some way.

You will need to be able to analyze the concepts and perhaps more importantly, analyze your own thinking about what you are learning. This is called "metacognition" and is crucial to critical thinking.


You need to stop and think about how you are thinking. We all have extremely complicated personal histories and established preconceptions. It is important to consider how your own preconceptions are coloring what you are learning. This is necessary to be able to objectively analyze and assess what you are learning. Metacognition is also important in nurturing creativity. It may seem that creativity resides in the arts, but science also depends on people being able to think along new and novel lines.


Science is not just a collection of facts to be memorized. It is a living body of ideas and concepts. Astronomy is in its golden age right now. Our telescopes are evolving dramatically and showing us things about the universe we could not see even a few years ago. The realm of computation is also growing incredibly fast, allowing us to model systems of increasing complexity.


Please keep all of these ideas in mind as you begin to study astronomy. First, read and study with your focus on understanding concepts, not memorizing facts. When you are working through homework questions, if you catch yourself feeling frustrated searching for a sentence in the reading that is the answer to your question, stop to think about learning itself. Perhaps you are trying to "find the answer" instead of understand it.

constellations in the night sky