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Transcript
Earth’s Motions Earth’s Motions – Big Questions: How can we make a model to help us understand our universe? What are the motions of the Earth? How do we know? How do the motions of the Earth affect our observations? Yes, the Earth really is a SPHERE! http://testtube.com/dnews/how-we-know-the-earth-isn-tflat/ Earth’s Motions There are three major motions of Earth Rotation Revolution Precession ROTATION Rotation Earth turning or spinning on an axis How does the sun appear to move in our sky? WHY? Because the Earth turns on its axis from the West to the East (counter-clockwise)! Effects of Earth’s Rotation… We experience day as our location on Earth turns to the sun and night as it turns away from the sun When we look south, the sun, stars, moon, planets, and & constellations all appear to rise in the East and set in the West Why do the star trails in the southern sky look different than the ones in the northern sky?? Because the Earth’s axis is tilted toward Polaris, so stars in the north appear to circle around that axis of rotation. Effects of Earth’s Rotation: Coriolis effect Because the Earth is wider at the equator, Earth rotates faster at the equator than it does at the poles In the Northern Hemisphere winds, weather patterns, water currents, airplanes, etc. all appear to deflect to the right because the Earth is moving underneath them. https://www.youtube.com/watch?v=mPsLanVS1Q8 Effects of Earth’s rotation… Plane flights Space flights pilots and navigators must compensate for objects veering to the right Creates special issues for astronauts in space because they move at a different rate than Earth Because of Earth’s Rotation Winds Distribution of Rain Uneven heating of the atmosphere causes winds blow from areas of high pressure to areas of low pressure If not for the deflection caused by the rotating Earth, the equator would get constant rain and north & south would be deserts. Evidence the Earth Rotates: The Foucault Pendulum A ball swinging back and forth on a string is called a pendulum. In 1851, French scientist Léon Foucault observed that his pendulum seemed to change direction over time. What force was causing this change in direction? http://www.youtube.com/watch?v=C3-uFao4dxQ The Foucault Pendulum Forces acting on a pendulum: Inertia causes it to swing straight out (Newton’s First Law of Motion) Gravity pulls it back down (Newton’s Law of Gravity) Air resistance makes it swing in shorter arcs over time (friction is a force that acts against inertia) So what makes the pendulum change directions? Pendulum as Evidence that Earth rotates: There is no force acting on the pendulum to make its plane of oscillation rotate The pendulum seems to change its path during the day, but actually it is the floor beneath it that changes, being twisted around by the daily rotation of earth. At the north pole the apparent rotation would be a full circle of 360 degrees each day, or about 15 degrees per hour Evidence of Earth’s rotation Astronauts have actually observed Earth’s rotation from space! https://www.youtube.com/watch?v=eqCyHeJzO4s All Planets Rotate: Rotation times: Mercury…………………58 Earth days Venus………………….. 243 Earth days Earth……………………23 hours 56 minutes Mars…………………….24 hours 37 minutes Jupiter…………………..9 hours 50 minutes Saturn…………………..10 hours 14 minutes Uranus………………….18 hours Neptune…………………18 hours 10 minutes Very similar Which two planets have the longest periods of rotation? Which planet has the shortest? Which planet’s period of rotation is almost the same as Earth’s? REVOLUTION Revolution Earth revolves around the Sun at an average speed of about 27 km/s or about 66,000 mph! One trip around the sun on Earth’s elliptical orbit equals one year. Earth’s Revolution in action… http://www.classzone.com/books/earth_science/terc/cont ent/visualizations/es0408/es0408page01.cfm?chapter_no= 04 Earth’s Orbit We are closest to the Sun at perihelion on January 3rd (147 million kilometers) We’re a little further from the Sun at aphelion on July 4th (152 million kilometers) Observational Evidence of Earth’s Revolution: Changing Constellations As earth moves along its orbit, we face the sun from a different perspective and we view different areas of space at night. Most of the constellations we can see in the summer are not visible in the night sky during the winter. This is evidence that Earth has changed locations in space! All planets revolve around the sun All Planets revolve around the sun near the same ecliptic plane as Earth. Remember Kepler’s third law… outer planets have longer orbital periods than inner planets do. Mercury………………...……….88 days Venus……………………………225 days Earth…………………………….365.25 days Mars…………………………….687 days Jupiter…………………………..12 years Saturn…………………………..29.5 years Uranus………………………….84 years Neptune………………………..165 years Understanding Orbital Motion The universal law of gravity allows us to understand orbital motion of planets and moons: Earth and moon attract each other through gravitation. Since Earth is much more massive than the moon, the moon’s effect on Earth is small. The moon’s inertia would cause it to move in a straight line (V), but… v Earth’s gravitational force constantly Moon accelerates the moon towards Earth (F) Gravity is constantly changing the moon’s direction of motion (Dv) , holding it on its almost circular orbit. Dv v’ F Earth Video : gravity and orbits http://svtube.svsd.net/play?id=uzptg84xwc Precession Precession Earth’s “wobble” on its axis Takes ~26,000 years to complete one wobble What causes Precession? The Earth is slightly oblate it bulges in the middle ~0.34% larger at equator due to its rotation and the fact that it is not completely rigid. The Sun and Moon have non-uniform gravitational forces on the Earth, pulling harder on the near side of the bulge than on the far side. (gravity weakens with distance) Consequences of Precession Polaris marks the north pole today Thuban was the pole star in 3000 BC Vega was in 12000 BC Polaris will become the north star again around 27800 AD Earth’s motions and TIME A year, a day, a month… A year is linked to Earth’s revolution (one orbit) A day is linked to Earth’s rotation (one spin) A month is linked to the cycle of the moon BUT… each of these things can be defined in different ways. How do you know when it’s been one full rotation? One full orbit? One full moon cycle? For example: Sidereal Day vs. Solar Day sidereal (or star) day: It takes the earth 23 hours and 56 minutes to rotate once with respect to the stars. This is one sidereal (or star) day. solar day: During that time, the earth has moved a bit along its orbit around the Sun. So… It takes about 4 minutes longer for the Sun to return to the same position in the sky Because we use the solar day, the stars rise and set 4 minutes earlier each day. Orbital Motion in One Solar Day Earth’s Motions – Big Questions: How can we make a model to help us understand our universe? What are the motions of the Earth? How do we know? How do the motions of the Earth affect our observations?
