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Background For Teachers:
A Greco-Egyptian astronomer named Claudius Ptolemaeus, known as Ptolemy, refines Aristotle’s ideas in the second century A.D. that Earth was the center of the universe. People remained feeling special, happy and content because it’s only right that everything should revolve us. Besides, there are references in the Bible that seem to support the idea of geocentricity.
For thousands of years this theory was so widely accepted -- it became accepted truth. It wasn’t until the mid 1500’s that a Polish astronomer named Nicholas Copernicus challenged the idea of a geocentric system because it did not explain the planetary motions. He developed a theory that Earth and the other planets orbit the Sun and is heliocentric (“Sun-centered”). People were angry at this idea because they lost the center of attention. However, Copernicus never had to deal with the wrath of the general populous because he died right after his ideas and theories were published.
In the early 1600’s, an Italian astronomer Galileo Galileo, with his telescope and observations of Jupiter’s moons and Venus’s phases proved that Copernicus was correct. The Sun is at the center of our system of planets, moons, and all heavenly objects orbit around it. However the public outrage was incredible! Nearly everyone accepted the Church’s teachings that the Sun orbits Earth. Even with evidence to support a Sun-centered system, Galileo was put on trial for publishing his ‘heretical’ observations. Under threat of torture (and Galileo is now quite old these days), renounced his ideas and spent the rest of his life under house arrest.
Our Solar System consists of a star, our Sun, nine orbiting known planets and their moons, asteroids, meteoroids, and comets. Our Sun, like other stars, is a huge ball of burning gases that produces tremendous amounts of heat and light. The Sun contains 99.86 percent of the mass of the entire solar system. Because of its mass, it gravitational force is able to keep the planets and all other objects in our Solar System in their elliptical orbit. Scientists believe that energy from the Sun is transferred through space in waves or magnetic energy known as the electromagnetic spectrum.
Planets are large heavenly bodies that orbit a star, in our case it’s the Sun. The word planet comes from the Greek word meaning ‘wanderer’. Ancient astronomers did not know how to explain how these moving objects could change their position in the heavens. The planets are at times referred to as the inner planets consisting of Mercury, Venus, Earth, and Mars. Each of these terrestrial (‘land’) planets have a rocky core and are extremely small in comparison to the giant gas planets. The giant gas planets are known to be the outer planets and are both massive in size, and mostly made of gases with a small solid core. Then there’s Pluto. Not necessarily an inner planet, but it is considered another small, terrestrial rocky planet.
Moons are natural satellites that orbit planets. Every planet has at least one moon except the planets Mercury and Venus.
Asteroids! There’s tens of thousands of them tumbling and bumping lumps of rock orbiting the Sun. Asteroids are also known as ‘minor planets’ or ‘planetoids’ and get their name from the Greek word that means ‘starlike’. When seen through a telescope, an asteroid looks like a faint star. There is an enormous gap of space in between the inner planets and the outer planets which consists of the ‘Asteroid Belt’. The vast majority of asteroids orbit the Sun between Mars and Jupiter. But they can also inhabit different regions of the solar system. Asteroids range in size from less than one kilometer in diameter to hundreds of km. They are usually irregular in shape and bumpy, not smooth and round like larger planets. Their surface is pockmarked with craters because over time they eventually crashed into one another. They split apart into smaller and smaller fragments leaving dust and smaller asteroids in the Asteroid Belt. Asteroids are made mostly of different types of rock, often rich in iron and other metals, and some ice. Since many meteorites found on Earth contain similar materials, astronomers believe that most meteorites are broken bits and pieces of asteroids that land on Earth.
Meteoroids are fragments from asteroids that have collided and split apart in small bits and pieces. These asteroid collisions send the meteoroids flying off in all directions across the Solar System. Some zoom toward Earth but the friction from our atmosphere makes the meteoroids so hot that they burn up in our atmosphere making a fiery streak of light called a meteor. People have referred to these ‘falling stars’ or ‘shooting stars’. When a meteoroid survives its hot journey through Earth’s atmosphere and falls to the ground, it is called a meteorite.
Comets have been referred to by astronomers as ‘dirty snowballs’ and ‘as close to nothing as something can get’ because they seem to be nothing more than rock dust wrapped around a spongy ball of ice. Throughout history there have been many superstitions about comets and many people were terrified calling them ‘terrible stars’ and ‘death-bringing stars’. Other people thought they were a sign of good fortune sent from the gods. Comets are made up of extremely small quantities of very simple chemicals like water, carbon dioxide, ammonia, methane, and space dust. They have a small head, called a nucleus and as it travels in its orbit toward the Sun, it slowly melts and releases space dust and gases. The space dust reflects light from the Sun looking like a giant ponytail in the sky. This is how comets originally got their name, from a Greek word meaning ‘longhair’. As a comet nears the Sun, it warms up. Ice evaporates from its surface and forms a large, tenuous cloud called a coma and the gases released form a yellowish dust tail and a bluish gas tail. The tail of a comet can stretch out for millions of kilometers. As the comet loops around the Sun and then speeds away in the same direction of its orbit, its tail is ahead of the comet rather than behind. Coming or going, the comet’s tail always points away from the Sun. Astronomers have studied enough comets to be sure that these visitors are members of the Solar System, just very distant members. Comets seem to have ‘home ground’ in the farthest reaches of the Solar System beyond Pluto. The home ground is a huge region called the Oort Cloud. The gravitational force of a passing star can snatch a comet from the Oort Cloud and fling it into a cigar-shaped elliptical orbit that brings it close to the Sun. Often times, comets will be pulled into Jupiter’s gravitational pull and never return to the Oort Cloud. This was the case in 1994, when Comet Shoemaker/Levy 9 rammed into Jupiter. Jupiter’s powerful gravity pulled the comet apart, so the planet was pelted by almost two dozen impacts.
A force is needed to cause something to move in a curved path, the planets and all heavenly objects, are no exception to this rule. A force of some kind must be acting to hold them in their orbits around the Sun. Isaac Newton connected this concept with the same force that pulls objects to Earth’s surface. Newton theorized that the moon revolves around the Earth. He called this phenomenon gravity and described it as an attractive force between any two objects. The strength of the force is related to the object’s mass (which is the amount of material an object contains).
The force of gravity attracts objects to each other. This attraction is not noticeable unless one of the objects is very large, such as a planet. The area within which gravity has an effect is called a ‘gravitational field’. The Earth and Moon both have gravitational fields, although the Earth’s is many times greater than the Moon’s because it is a much bigger object.
The strength of the pull of gravity between two objects also depends on how far apart they are and their masses. Weight is the measure of the pull of gravity on an object’s mass. The further away an object is from the center of the Earth, the less pull gravity has on it. Gravity is a fundamental force in the sense that is cannot be explained in terms of any other force. Gravitational forces act between all bodies everywhere and hold planets, their moons, stars, and galaxies together.
Intended Learning Outcomes:
Why do planets stay in orbit? This activity will help make the connection between gravitational force and orbital motion. Participants will make observations, record discoveries, and graph data as they change different variables: such as, the size and mass of different corks and rubber stoppers, and length of string.
Activity #1 – Oops! There It Goes!
Activity #2 – My Very Energetic Mother Just Served Us Nine Pizzas (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto)?
Staying Up While Falling Down
This activity demonstrates how a satellite (natural or man-made) stays in orbit by modeling the effects of Earth’s gravity on a satellite and the Sun’s gravity on the planets. Participants will observe gravity acting as a centripetal “center-seeking” force.
Writing A Persuasive Paper - Is Pluto A Planet?
This writing activity integrates reading comprehension, the writing process, and science skills of logical reasoning and using evidence to support a particular conclusion.
National Research Council (2000). Inquiry and the National Science Education Standards—A Guide for Teaching and Learning
“Inquiry-based instruction is a multifaceted activity that involves making observations; posing questions; examining books and other sources of information to what is already know; planning investigations; reviewing what is already known in light of experimental evidence; using tools to gather, analyze, and interpret data; proposing answers, explanations, and predictions; and communicating the results. Inquiry requires identification of assumptions, use of critical and logical thinking and consideration of alternative explanations.”
Freedman, R.L.H. (1994). Open-ended Questionings—A Handbook for Educators
“Student-centered authentic assessment occurs when critical-thinking skills,, the writing process, and content-area instruction are combined and used in open-ended questions. By their nature, open-ended questions assess writing, conceptual understanding, and thinking skills--especially students’ abilities to analyze, to evaluate, and to solve problems.”
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