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# Heliocentric: The Movement of the Earth Around the Sun (page 2)

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Author: Janice VanCleave

1. Map the apparent path of the Sun across the sky by placing an X in the center of a 12-inch (30-cm) square of cardboard. At sunrise or early morning, place the cardboard outdoors where it will receive direct sunlight during the entire day. Place the bowl from the original experiment upside down on the cardboard with the X in the center of the bowl. Starting at or near the time of sunrise, touch the glass dome with the tip of a pencil so that the shadow of the pencil's tip falls on the X mark. With the tip of a marking pen, make a dot on the glass where the tip of the pencil touches the glass. Continue making marks every hour or as often as possible throughout the day. Use a compass to determine the direction ofthe Sun's apparent movement.
2. Repeat the previous experiment during different months. Take photos of the bowl after each experiment. Date the photos and use them to represent the changes in the apparent movement of the Sun during different times of the year.
1. Claudius Ptolemy was one of the great astronomers. Little is known about his life, including his birth date, but he worked in Greece around A.D. 140. It was at this time that he proposed a geocentric (Earth-centered) model for our solar system. According to Ptolemy, the Sun, the Moon, and the planets each move in small circles called epicycles. The centers of the epicycles trace out larger circles, called deferents, around the Earth. Near the center of all these larger circles is the Earth. Make a diagram similar to Figure 4.2 showing Ptolemy's model of the geocentric solar system. Note that a line drawn from the Earth to the Sun would pass through the centers of the epicycles of Mercury and Venus. This is how Ptolemy explained the fact that these planets always appear near the Sun.
2. Sir Isaac Newton (1642–1727), an English scientist, determined that planets move in orbits around the Sun because of gravitational attraction between the Sun and each planet. Model how gravity (the force that pulls celestial bodies toward each other) keeps planets and other satellites in orbit around the Sun. Lay a cookie sheet on a table. Place a toilet tissue tube in one comer of the cookie sheet so that one end of the tube rests on the rim of one short side of the pan. Secure the raised end of the tube to the rim of the pan with tape. Lay a piece of typing paper in the pan so that the untaped end of the tube rests on the edge of the paper. Prop up the long side of the pan nearest the tube about 1 inch (2.5 cm) by placing a lump of clay under each comer of the long side. Fill a cup one-quarter full with water, and add 10 or more drops of red food coloring. Stir. Wet a marble with the colored water, place the marble in the tube, and release it. The marble follows a curved path because it has two directions of motion, as shown by the arrows in Figure 4.3: forward because of the slanted tube and down because gravity pulls it down the raised pan. Draw the force arrows on the paper. Photograph the setup before and after releasing the marble. Use the photographs and the paper to show the direction of the forces on the marble and the resulting curved path.

### Get the Facts

In 1543, Nicolaus Copernicus (1473–1543), an astronomer of German and Polish descent, proposed the heliocentric theory. Find out about and make a diagram of Copernicus's heliocentric model.

Johannes Kepler (1571–1630), a German astronomer, discovered that planets have elliptical (oval) orbits. He also showed that the Sun is not in the center of a planet's elliptical orbit but closer to one end of the ellipse than the other. Find out more about Kepler and the experiments that led him to his discoveries.

Galileo Galilei (1564–1642), an Italian astronomer and physicist, used a telescope he made to observe that Venus passes through the same phases as does the Earth's Moon. Find out how this discovery and others made by Galileo support the heliocentric theory.

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