The Earth's Movements (1972)

[Music] hello [Music] on a day early in july about twenty thousand miles above the sunlit side of the earth our home planet would look something like this a sphere in space apparently motionless but we know the earth is really moving one sign of this is the movement of the sun across the sky on earth in new york the sun rose an hour ago on a cool july morning it is still low in the sky 3 500 miles to the east in london it is nearly noon farther east in a village in india at the same moment the sun has begun to sink toward the west in japan the sun is setting the sun rises and sets over all parts of the world because of one of the earth's movements because of another movement too slow for us to see the seasons change on this july day in the northern hemisphere there is a pond that is a favorite swimming spot but it's a skating pond six months later in january many seasonal and other changes take place on earth because of the earth's movements you're going to learn about four of these movements if we could speed up one of them several thousand times you'd see the movement we call rotation the turning of the earth on its axis a marker on the earth changes positions as we watch it turns toward us and away from us and then finally will disappear from view in the same way all parts of the earth turn past the sun eventually the marker will appear again from the opposite direction the earth makes one complete rotation in approximately 24 hours what happens when the earth's rotation turns the marker away from the sun by looking down at the rotating earth from over the north pole we can see the marker moves from sunlight into shadow from day into night all parts of the world go through periods of light and darkness because of the earth's rotation of course to people on earth it's the sun that seems to be in motion it rises reaches its highest point in the sky sets then rises again so at the same moment to people in different parts of the world it is a different time of day so for instance at the center of the lighted half the sun appears at its highest point elsewhere here for instance the sun is rising it is dawn here while at this same moment halfway around the world here the sun is setting the day is ending so you can see why at different points around the world it is a different time of day as the earth rotates the time of day continually changes for all parts of the world those differences in time weren't important long ago when transportation and communication were primitive then people traveled only short distances in a day today rapid worldwide transportation and nearly instant communication have made us more aware of time differences in the different parts of the world today we can tell what time it is in any part of the world because the nations of the world have adopted a uniform system of time zones time zones of the earth are based on the fact that every part of the world turns through a full circle of 360 degrees every 24 hours this means that in one hour the earth rotates through 1 24th of a circle 15 degrees based on this the earth has been divided into 24 time zones each 15 degrees wide each of these dark and light bands represents a time zone in general time in one zone is exactly one hour later than in the next zone to the west and exactly one hour earlier than in the next zone to the east the united states extends across seven time zones from eastern maine to hawaii and the western tip of alaska remember we've imagined the earth's rotation has sped up several thousand times to see a second movement of the earth we have to imagine that its speed is increased even more and move further away in space now with the sun in the distance we see the movement called the earth's revolution the earth revolves around the sun traveling through space in a nearly circular orbit now six months later we look past the sun and see the earth in the distance continuing its revolution around the sun in 12 months or more exactly 365 and one-fourth days after we first began watching the earth will have made one complete revolution around the sun the time it takes the earth to make one revolution of the sun is the period of time we call a year the earth makes one revolution every year the earth's revolution is part of the reason we have seasons to see why let's think about the imaginary line around which the earth rotates the earth's axis it is not straight up and down perpendicular to the plane of the earth's orbit the axis is tilted at an angle if we projected the earth's axis out into space it would point toward polaris the north star at an angle of twenty three and a half degrees from the perpendicular because of this angle the earth as it revolves around the sun does not receive the sun's rays uniformly throughout the year beginning about june 21st the northern hemisphere has warmer weather and longer days because it is tipped toward the sun as the earth rotates the northern hemisphere is within the sun's light a longer part of each 24-hour period it is summer in the north now as the earth revolves around the sun the tilt or inclination of its axis remains the same so when the earth has revolved to the opposite side of its orbit conditions are reversed now around december 21st the northern hemisphere is tilted away from the sun it is winter there but the southern hemisphere is now tilted toward the sun now as the earth rotates the southern hemisphere is within the sun's light a longer part of each 24-hour period it is summer in the southern hemisphere and winter in the northern hemisphere and so two movements of the earth rotation and revolution plus the tilt of the earth's axis combined to produce the seasons of the year to see a third movement of the earth let's think of the rotating earth as a spinning gyroscope notice the wobble of its axis the earth's axis wobbles too because of the pull of gravity of the sun and moon on the earth one completion of this conical path of the axis takes about twenty six thousand years one effect of this we won't see is that in about twelve thousand years the earth's axis will point toward the star vega and our descendants will call it the pole star this third movement is the precession of the equinoxes the wobble or variation in the direction of the earth's axis the three movements we've seen involve just the earth as it moves through its orbit around the sun a fourth movement takes place in a much faster dimension to understand it we must remember that our sun is only one of billions of stars that form a great spiral system a galaxy our galaxy the milky way looks something like this the sun's position in the galaxy is about here the galaxy rotates like an enormous wheel and our solar system moves with it making one revolution every two hundred million years this galactic movement is a fourth movement of the earth rotation revolution precession of the equinoxes and galactic movement four movements of the earth today one of the purposes of space exploration is to understand more fully and more accurately all the movements of the earth