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# Line Up: What Did People in the Past Think the Universe Looked Like?

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Source:
Author: Janice VanCleave

### PROBLEM

What did people in the past think the universe looked like?

### Materials

• yardstick (meterstick)
• 22-by-28-inch (55-by-70-cm) piece of poster board
• pencil
• 26-inch (65-cm) piece of string
• school glue
• 2-inch (5-cm) Styrofoam craft balls (available at craft stores)
• marking pen

### Procedure

1. Lay the yardstick (meterstick) across the middle of the poster board, parallel with the long sides.
2. Using the pencil, make nine small dots on the poster board next to the yard-stick, one every 3 inches (7.5 cm) from the end. The last dot will be 1 inch (2.5 cm) from the edge of the poster board.
3. Tie a loop in one end of the string.
4. Place the pencil point through the loop and stand the point on the second dot from the left side of the poster board. Pull the string outward to stretch it over the first dot. Hold the string on the first dot with your thumb as you move the pencil point across the poster board to draw the largest part of a circle.
5. Repeat step 4 for each of the remaining dots.
6. Glue one Styrofoam ball to each of the first eight dots.
7. Using the marking pen, add labels as shown, and draw stars in the space beyond Saturn.

### Results

You have made a model that shows how people in the past thought the universe was organized.

### Why?

In this experiment, a geocentric (Earth-centered) model of the universe (Earth and all other natural objects in outer space) is made. This model was proposed by the ancient astronomer Ptolemy, who lived and worked in Alexandria, Greece, in the second century A.D. Ptolemy believed that Earth was motionless and that the Moon, the planets, and the Sun revolved (moved around a center point) around Earth. An outer dome of stars was thought to exist beyond the farthest planet (then thought to be Saturn). Ptolemy believed that in relation to their starry background, the Sun and the Moon continued to move along their paths around Earth, while other celestial bodies (natural objects in the sky) seemed to wander. These wandering celestial bodies were called planets, from the Greek word for wanderers. Today's model of the Sun and its planets (celestial bodies that move in a path around a sun) is heliocentric (sun-centered) with nine planets.

### LET'S EXPLORE

Nicolaus Copernicus (1473–1543), a Polish astronomer, proposed a heliocentric model of the universe. In Copernicus's model, the celestial bodies were in the following order from the central, stationary Sun: Mercury, Venus, Earth with the Moon as a satellite (a body revolving around another body), Mars, Jupiter, and Saturn. Use the same materials as in the previous experiment to show Copernicus's model of the universe.

### SHOW TIME!

Mercury is 0.4 AU from the Sun.

Prepare a table similar to the one shown here, giving the average distances of the planets in whole millions of miles (km) from the Sun (see the appendix) and the AU distances calculated to the nearest decimal place, as in the previous example.

Using the scale of 1 cm: 1 AU, draw a line 39.7 cm long on a paper to represent 39.7 AU, the distance between the Sun and Pluto, the outermost known planet in our solar system. Place a small circle with rays at the left end of the line to represent the Sun and a dot at the opposite end for the planet Pluto. Using the AU distances of each planet, place a dot on this line for each planet according to the scale. Label each planet.

1. The first models of the universe were mainly models of our solar system. A solar system is a group of celestial bodies that orbit (move in a curved path around) a star called a sun. A modern model of our solar system shows Earth as one of the nine known planets that revolve around the Sun. Make a drawing that depicts the modern concept of the solar system.
2. The average distance between Earth and the Sun is 93 million miles (149 million km). This distance is called an astronomical unit (AU) and is used as a measure of distance in the solar system. The AU distance for any planet can be calculated by dividing the planet's average distance in miles (km) by 93 million miles (149 million km). For example, the average distance of Mercury from the Sun is 36 million miles (58 million km). Thus, the AU distance between the Sun and Mercury can be calculated as follows:
• 36 million miles ÷ 93 million miles = 0.4 AU
• 58 million km ÷ 149 million km = 0.4 AU
3. Use the AU distances to make a scale model of the distances of the planets from the Sun. A scale model is a model made in proportion to the object or objects that it represents. For example, a scale of 1 cm : 1 AU means that 1 cm on the model represents 1 AU.

### CHECK IT OUT!

When Copernicus proposed a heliocentric model of the universe, it was not widely accepted. About 70 years after Copernicus's death, Galileo Galilei (1564–1642), an Italian astronomer and physicist, used a telescope he made to study the heavens. His observations supported Copernicus's heliocentric idea. Find out more about Galileo's observations. Were Galileo's ideas about the universe accepted?

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