LEARNING ACTIVITY SHEET Kepler's Law of Planetary Motion Background Information for the Learners (BIL) In the early 1600s, Johannes Kepler proposed three laws of planetary motion. Kepler was able to summarize the carefully collected data of his mentor - Tycho Brahe - with three statements that described the motion of planets in a sun-centered solar system. Kepler's efforts to explain the underlying reasons for such motions are no longer accepted; nonetheless, the actual laws themselves are still considered an accurate description of the motion of any planet and any satellite. Kepler's three laws of planetary motion can be described as follows: The path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus. (The Law of Ellipses) An imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time. (The Law of Equal Areas) The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun. (The Law of Harmonies) The Law of Ellipses Kepler's first law - sometimes referred to as the Law of Ellipses - explains that planets are orbiting the sun in a path described as an ellipse. An ellipse is a special curve in which the sum of the distances from every point on the curve to two other points is constant. The two other points (represented here by the tack locations) are known as the foci of the ellipse. The closer together that these points are, the more closely that the ellipse resembles the shape of a circle. In fact, a circle is the special case of an ellipse in which the two foci are at the same location. Kepler's first law is rather simple - all planets orbit the sun in a path that resembles an ellipse, with the sun being located at one of the foci of that ellipse.The Law of Equal Areas Kepler's second law - sometimes referred to as the Law of Equal Areas - describes the speed at which any given planet will move while orbiting the sun. The speed at which any planet moves through space is constantly changing. A planet moves fastest when it is closest to the sun and slowest when it is furthest from the sun. Yet, if an imaginary line were drawn from the center of the planet to the center of the sun, that line would sweep out the same Law of Equal Areas area in equal periods of time. For instance, if an imaginary line were drawn from the earth to the sun, then the area swept out by the line in every 31-day month would be the same. This is depicted in the diagram below. As can be observed in the diagram, the areas formed when the earth is closest to the sun can be approximated as a wide but short triangle; whereas the areas An imaginary line drawn from the sun to planet sweeps out equal areas in equa formed when the earth is farthest from the amounts of time. sun can be approximated as a narrow but Visit this page for the animation: long triangle. These areas are the same https:/www.physicsclassroom.com/class/circles/Le 4/Kepler-s-Three-Laws size. Since the base of these triangles are shortest when the earth is farthest from the sun, the earth would have to be moving more slowly in order for this imaginary area to be the same size as when the earth is closest to the sun. The Law of Harmonies Kepler's third law - sometimes referred to as the Law of Harmonies - compares the orbital period and radius of orbit of a planet to those of other planets. Unlike Kepler's first and second laws that describe the motion characteristics of a single planet, the third law makes a comparison between the motion characteristics of different planets. The comparison being made is that the ratio of the squares of the periods to the cubes of their average distances from the sun is the same for every one of the planets. As an illustration, consider the orbital period and average distance from sun (orbital radius) for Earth and mars as given in the table below.Period Average T2/R3 Planet (s) Distance (m) ($2/m3) Earth 3.156 x 10 s 1.4957 x 1011 2.977 x 10-19 Mars 5.93 x 10's 2.278 x 1011 2.975 x 10-19 Observe that the T2/R3 ratio is the same for Earth as it is for mars. In fact, if the same T2/R$ ratio is computed for the other planets, it can be found that this ratio is nearly the same value for all the planets (see table below). Amazingly, every planet has the same T2/R3 ratio. Period Planet Average T2/R3 (yr) Distance (au) (yr2/au3) Mercury 0.241 0.39 0.98 Venus 615 0.72 1.01 Earth 1.00 1.00 1.00 Mars 1.88 1.52 1.01 Jupiter 11.8 5.20 0.99 Saturn 29.5 9.54 1.00 Uranus 84.0 19.18 1.00 Neptune 165 30.06 1.00 Pluto 248 39.44 1.00 (NOTE: The average distance value is given in astronomical units where 1 a.u. is equal to the distance from the earth to the sun - 1.4957 x 101 m. The orbital period is given in units of earth-years where 1 earth year is the time required for the earth to orbit the sun - 3.156 x 107 seconds. ) domash to Kepler's third law provides an accurate description of the period and distance for a planet's orbits about the sun. Additionally, the same law that describes the T2/R3 ratio for the planets' orbits about the sun also accurately describes the T2/R3 ratio for any satellite (whether a moon or a man-made satellite) about any planet. There is something much deeper to be found in this T2/RS ratio - something that must relate to basic fundamental principles of motion.Activity 3: Count Me (2nd Law) Directions: Look at the diagram below. Count the number of squares in sector 1 and in sector 2, then place this diagram and your count in your notebook. Squares in Sector 1: Squares in Sector 2: 10 months Sector 2 Sector 10 month Source: Henderson, The Physcis Classroom. hlips //www.physicsclassroom.com/glass/circles/Lesson-4/Kepler-s-Three-Laws Guide Questions: 1. What can you say about the number of squares in Sector 1 compared to the number of squares in Sector 2? 2. What does the number of squares implies?3. If it takes the same amount of time for a planet to move from point A to point B as it does for a planet to move from point C to point D, then what must a planet do in terms of its speed in each sector? 4. Earth's perihelion is in January and its aphelion is in July. Why is this not the reason for the season on Earth? If it was, the Northern Hemisphere on Earth would be hotter in January and colder in July. Think about it. Activity 4: Kepler's Third Law of Planetary Motion Directions: Study the table below and answer briefly the questions given based from your analysis. Planets Mean Orbital Velocity and Mean Distance to the Sun Mercur Venu Earth Mars Jupite Satur Uranu Neptun Pluto y n S Velocity (km/s) 47.87 35.0 29.7 24.1 13.07 9.67 6.84 5.48 4.75 2 9 3 Distanc 0.39 0.72 1.00 1.52 5.20 9.54 19.19 30.07 39.4 8 e ( AU ) 1. How does the distance from the Sun of a planet affect the planet's orbital velocity?2. What do you think is the reason behind the relationship you concluded on the first question? 3. Based on your response to number 1, what do you think Kepler's Third Law of Planetary Motion say? Reflections: 1. I learned that 2. I enjoyed most on 3. I want to learn more on