In our world of technology, when we think about measuring distances to far away bodies we often don’t remember that early civilizations like the Greeks were able to use simple techniques to measure long distances. This activity pushes you to consider how you might measure the distance to a planet with materials around you.

Imagine that this year, a new object was observed in the night sky. Six months ago it appeared as an object glowing with medium brightness, near Cassiopeia. Lately it appears brighter in the night sky, and shows up in the middle of the Cassiopeia constellation. Astronomers believe this object is traveling closer to Earth. But how fast is it traveling, and how far away is it now?

Get into small groups and brainstorm how you might measure the distance to this new observed object. Use the Measuring Great Distances Worksheet to keep track of all your ideas. You don’t need to select one idea now; in fact, you need all of the creative ideas you can think of!

Sometimes the answer to one question lies within the answer to another question. In this case, astronomers first calculated the distances to the Moon and Venus before being able to calculate the distance to the Sun. You can observe parallax with your own eyes and remember that this perceivable shift is what helped ancient astronomers first calculate great distances.

This article gives an overview of the idea of parallax, and how ancient Greek astronomers estimated the distance to the Sun without telescopes or other modern technology.

Before you dive into the articles, review the Big History Reading Guide with your class. Then, look at the Three Close Reads Worksheet and review the directions. Think about the difference between approaching readings for capturing gist, reading for factual information, and analyzing for conceptual thinking.

Skimming for Gist

Ancient astronomers wondered how far away the Sun was, but had no modern equipment to help them measure that distance. Astronomers used other objects in our Solar System, and a method called parallax, to calculate the distance to the Sun.

Understanding Content

By the end of the second close read, you should be able to answer the following questions:

1. What is parallax?
2. What phase of the Moon did Aristarchus use to estimate Earth’s distance from the Sun?
3. Why is the 1769 transit of Venus so important to our current understanding of the Universe?
4. What kind of a triangle did Aristarchus use to calculate the distance to the Sun?

Thinking Conceptually

Could you use parallax to measure shorter distances? For example, to an object on Earth?

Ancient Greek astronomers calculated the circumference of the Earth, and were then able to find distances to the Moon and Sun. The distance to the Sun is called the astronomical unit, which astronomers use, along with parallax, to calculate our distance to stars. The distance to stars that are very far away can be found by combining spectroscopy—measuring the brightness of stars—with known distances to nearer stars.

This video will explain some of the earliest astronomical observations and calculations that helped us determine the astronomical unit, which in turn allows us to calculate huge distances in our Solar System and throughout the Universe.

Preview

In this video, Phil Plaitt explains how the Greek astronomer Eratosthenes figured out the circumference of the Earth by observing the angle of a shadow at noon during the summer solstice. With this value known, later astronomers were able to estimate distances to the Moon and the Sun. The distance to the Sun is called the astronomical unit (AU), and was more precisely measured with the help of radio telescopes and Kepler and Newton’s orbital equation. Knowing the distance from the Sun allows astronomers to measure great distances in our Universe, using our orbit around the Sun as a baseline in calculations using parallax. The distance to the farthest stars are calculated by using spectroscopy to determine how bright they are, and then comparing their brightness to closer stars whose distance is already known.

Key Ideas—Factual

Think about these questions as you watch the video.

1. What enabled Eratosthenes to calculate the different kinds of shadows the Sun cast at noon during the summer solstice?
2. What distances does knowing the Earth’s circumference help us calculate?
3. What distance is the astronomical unit based upon?
4. Why did scientists use radio telescopes to bounce radar pulses off of Venus?
5. What does having two eyes enable us to do regarding distances?
6. What does the Earth’s orbit have to do with calculating distances to stars?
7. Why do we use measurements like light-years and parsecs?
8. What does spectroscopy—measuring the brightness of a star—have to do with its distance?

Thinking Conceptually

Now that you understand the concept of parallax, why do you imagine it’s important for astronomers around the world to collaborate on projects like measuring tremendous distances in our Universe?

Note: This activity is an adaptation of material © 1992 by the Regents of the University of California, published in the Project ASTRO Resource Notebook by the Astronomical Society of the Pacific.

In this activity, you will practice using parallax to measure an unknown distance in your classroom. 

Working in small groups, you’re going to practice using parallax to measure distances using tools similar to those Aristarchus and other early astronomers would have used. You will measure the distance to an object in the classroom using simple geometry.

All of the instructions for this activity are included in the worksheet. When you’ve completed all of your calculations, work with your group to answer the following questions:

1. How accurate were your calculations?
2. If you had the chance to do this activity again and you wanted to increase your accuracy, would you choose to increase or decrease the size of your baseline? Why?

Be prepared to share your answers with the class.

Please click here to watch this video. Note that this will take you off the BHP site.

Before scientists were able to prove the age of extremely old things, they had a number of creative ideas to help determine the age of the Earth. With our modern scientific equipment, we are now able to measure the radioactivity of meteorites. The currently-accepted age of the Earth and the entire Solar System is 4.54 billion years.

This video gives an overview of how we have come to know that the Earth is 4.54 billion years old. Modern scientists have measured the radioactivity of meteorites, which all show the same age of 4.54 billion years old. This helps to confirm that the Earth—and everything in the Solar System—was created at the same time from the solar nebula.

Preview

In this video, Fraser Cain explains the current accepted theory that the Earth—along with the rest of the Solar System—was formed 4.54 billion years ago, out of the solar nebula. Many scientists throughout history attempted to confirm the age of the Earth using a number of theories that all turned out to be wrong. Meteorites that landed on Earth provided the answer; scientists can measure the radioactive isotopes of space material to determine its age. Since all the meteorites tested showed the same age—4.54 billion years—this confirms that everything in the Solar System was created then.

Key Ideas—Factual

Think about the following questions as you watch the video.

1. What is our current estimate age of the Earth and the entire Solar System?
2. Why can’t we just measure the age of a rock on the Earth’s surface and call that the age of the Earth?
3. What are some creative ways scientists throughout history have tried to confirm the age of the Earth?
4. What did scientists measure in order to confirm the age of the Earth and the entire Solar System?

Thinking Conceptually

Carbon dating, also called radiocarbon dating, is a variety of radioactive isotope dating. Scientists use carbon dating to measure the age of matter that was once living. The half-life of carbon 14 is 5,730 years. Can you think of any old, once-living matter that we could carbon date?

Note: This activity is an adaptation of material produced by the Exploratorium Teacher Institute.

The video you watched in the last activity (How Old Is the Earth?) explained that the Solar System is over 4 billion years old. In this activity, you’ll learn about radioactive decay and the concept of half-life, which scientists use to measure extremely old things. 

Get into small groups. Your teacher will distribute a cup filled with 100 pennies to each group. As a group, do the following:

• Toss all 100 pennies onto the table.
• Remove the pennies that land tail-side up on the table, and place them flat on the table, arranged in a tall column.
• Gather up the remaining pennies and toss them again. Remove the pennies that land tail-side up and arrange them in a second column, next to the first column.
• Repeat until all the pennies have been removed.
• If no pennies come up tails on a toss, leave an empty column.

The chance that any penny will come up tails is always the same: 50 percent. This is why after each toss, approximately half of the pennies are removed.

In this model, the removal of a penny signifies the decay of a radioactive nucleus. The time it takes for half of the remaining pennies to be removed is called the half-life. Since each penny has a 50 percent chance of decaying (coming up tails), each time it’s tossed, the half-life of the pennies in this model is about one toss.

In the How Old Is the Earth?” video, you learned that scientists are able to measure very old things through measuring radioactive decay. Isotopes are chemical elements that have different numbers of neutrons in their nuclei, and they break down, or decay, at predictable rates. An isotope breaks down into what is called a decay product. The tail-side up pennies in this model are the decay product, and the head-side up pennies represent the remaining radioactive element.

Once we know the rate of decay of a radioactive element in an object (like the meteorites in the last video), we can determine its age. If an object is made up of 50 percent decay product, then the object has been through one half-life.

Count the number of columns you made during this first round of the experiment. Now, gather all of your group’s pennies and conduct the experiment again from the beginning. This time, plot the number of tail-side up pennies on the Modeling Radioactivity Worksheet.

Be prepared to report back to the class answers to these questions:

1. How many columns, or half-lives, did your pennies have in the first round of your experiment? How about in the second round?
2. Did you ever have exactly half of your pennies come up tails?
3. What if we did this experiment again, like this: Each student in class has a penny and everyone tosses their pennies at the same time. Those students whose pennies landed tail-side up, had to sit down. About how many students do you think would sit down after one half-life? After two half-lives? How many half-lives do you think your “radioactive” class has?

Have your completed Measuring Great Distances (Part 1) Worksheet available.

In this activity, you’ll revisit the challenge that was presented at the beginning of this lesson. Now that you’ve gained a more concrete understanding of how we measure great distances in the Universe, you’ll work in your small group to use that information to add ideas to your list, and then select a final method to measure the distance to this new glowing object in the sky.

In your group, you’re going to add to your list of ideas about how to measure the distance to this new object in the sky, and then select one final method for measuring it. Remember that scientists often collaborate with other groups of scientists to tackle projects like this.

Here’s what we know about the object: Six months ago, it appeared, glowing with medium-brightness, near Cassiopeia; more recently it appears brighter in the night sky and shows up in the middle of the Cassiopeia constellation. You need to come up with a way to measure the distance to this object to help determine if it’s traveling closer to Earth.

Work with your group to add to your brainstorming list about how you might measure the distance to this object. Copy your ideas from your Measuring Great Distances (Part 1) Worksheet onto the Measuring Great Distances (Part 2) Worksheet, and add any additional ideas to the Part 2 Worksheet. As a group, you need to decide on an idea and strategy for measuring the distance to this new object. Provide the following information on the new worksheet, along with a rationale for the method of measurement that you chose:

• Method to measure the distance to this object
• Any assistance you might need from another group of astronomers
• Length of time needed to complete the measurement process
• Why you decided on this method of measurement

Make sure your group is prepared to share these answers with the class.