3(iii). Movements of the Earth lecture.docx

3(iii). Movements of the Earth lecture.docx

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  1 3(iii). Movements ofthe Earth Main movements of the Earth are as under: A. Rotation of the Earth Earth's rotation is the rotation of Planet Earth around its own axis. Earth rotates eastward, in prograde motion. As viewed from the North Pole star Polaris, Earth turns counter clockwise. The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where Earth's axis of rotation meets its surface. This point is distinct from Earth's North Magnetic Pole. The South Pole is the other point where Earth's axis of rotation intersects its surface, in Antarctica. Earth rotates once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, and 4 seconds with respect to the stars. Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds. Analysis of historical astronomical records shows a slowing trend of about 2.3 milliseconds per century since the 8th century BC. At the equator, the circumference of the Earth is 40,070 kilometers, and the day is 24 hours long so the speed is 1670 kilometers/hour (1037 miles/hr). This decreases by the cosine of your latitude so that at a latitude of 45 degrees, cos (45) = .707 and the speed is .707 x 1670 = 1180 kilometers/hr. You can use this formula to find the speed of rotation at any latitude. True Solar Day is the Earth's rotation period relative to the Sun (solar noon to solar noon) is its true solar day or apparent solar day. It depends on the Earth's orbital motion and is thus affected by changes in the eccentricity and inclination of Earth's orbit. Mean solar day is the average of the true solar day during the course of an entire year is the mean solar day, which contains 86,400 mean solar seconds. Currently, each of these seconds is slightly longer than an SI second because Earth's mean solar day is now slightly longer than it
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  2 was during the19th century due to tidal friction. The average length of the mean solar day since the introduction of the leap second in 1972 has been about 0 to 2 ms longer than 86,400 SI seconds. Stellar and Sidereal day is the Earth's rotation period relative to the fixed stars, called its stellar day. On Earth, a sidereal day lasts for 23 hours 56 minutes 4.091 seconds, which is slightly shorter than the solar day measured from noon to noon. Our usual definition of an Earth Day is 24 hours, so the sidereal day is 4 minutes faster. This means that a particular star will rise 4 minutes earlier every night, and is the reason why different constellations are only visible at specific times of the year. User
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  3 Effects of theEarth’s rotation: The rotation of the Earth has several significant effects on various aspects of our planet and its inhabitants: 1. Day and Night Cycle: Earth's rotation causes the alternation between day and night. As the Earth rotates on its axis, different parts of the planet are exposed to sunlight, creating a 24-hour cycle. 2. Coriolis Effect: This effect, caused by Earth's rotation, deflects moving objects, such as air and water currents, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It influences global wind patterns, ocean currents, and the trajectory of moving objects like missiles and hurricanes. 3. Formation of Tides: Earth's rotation interacts with the gravitational pull of the Moon and the Sun to create tidal patterns in the oceans. The gravitational forces cause bulges of water on opposite sides of the Earth, resulting in high and low tides. 4. Weather Patterns: The rotation of the Earth plays a crucial role in shaping weather patterns. It contributes to the creation of prevailing winds, which in turn influence weather systems, such as the formation of cyclones, anticyclones, and the distribution of precipitation. 5. Equatorial Bulge: Earth's rotation causes it to bulge slightly at the equator, making the planet slightly wider around the middle than from pole to pole. This bulge affects various geophysical measurements and contributes to the shape of ocean basins and the distribution of landmasses. 6. Centrifugal Force: The rotation of the Earth generates a centrifugal force, which is stronger at the equator than at the poles. This force counteracts the gravitational pull and slightly reduces the force of gravity at the equator. 7. Navigation: Earth's rotation is essential for navigation using systems such as GPS (Global Positioning System). It enables accurate determination of position by calculating the time difference between signals sent by satellites and received on Earth.
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  4 8. Timekeeping: Earth'srotation provides the basis for our measurement of time. The concept of a day, defined as one complete rotation of the Earth on its axis, forms the foundation for calendars and timekeeping systems worldwide. Overall, the rotation of the Earth influences a wide range of phenomena, from the formation of weather patterns to the measurement of time, and it is fundamental to many aspects of life on our planet. B. Revolution of the earth Earth's revolution around the Sun takes much longer than its rotation on its axis. One complete revolution takes 365.26 days, or one year. The Earth revolves around the Sun because gravity keeps it in a roughly circular orbit around the Sun. The Earth's orbital path is not a perfect circle, but rather an ellipse, which means that it is like a slight oval in shape. This creates areas where the Earth is sometimes farther away from the Sun than at other times. We are closer to the Sun at perihelion (147 million kilometers) on about January 3rd and a little further from the Sun (152 million kilometers) at aphelion on July 4th. Students sometimes think our elliptical orbit causes Earth's seasons, but this is not the case. If it were, then the Northern Hemisphere would experience summer in January. During one revolution around the Sun, the Earth travels at an average distance of about 150 million kilometers. Mercury and Venus take shorter times to orbit the Sun than the Earth, while all the other planets take progressively longer times depending on their distance from the Sun. Mercury only takes about 88 Earth days to make one trip around the Sun. While Saturn, for example, takes more than 29 Earth years to make one revolution around the Sun. Earth revolves around the Sun at an average speed of about 27 kilometers (17 miles) per second. Our planet moves slower when it is farther away from the Sun and faster when it is closer to the Sun. The reason the Earth (or any planet) has seasons is that Earth is tilted 23 1/2 degrees on its axis. This means that during the northern hemisphere "summer" the North pole points toward the Sun, receiving direct solar rays, and in the northern hemisphere "winter" the North Pole is tilted away from the Sun and the rays of the Sun are angled rather than direct.
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  5 Thus, the seasonwe experience depends not on where the Earth is in its revolutionary orbit around the Sun, but rather the inclination of the axis of the Earth. This means "summer" in the northern hemisphere actually occurs when Earth is farthest from the Sun, but inclined toward it, and "winter" occurs when Earth is closest but inclined away. Events in the orbit: By astronomical convention, the four seasons are determined by the solstices (the two points in the Earth's orbit of the maximum tilt of the Earth's axis, towards the Sun or away from the Sun) and the equinoxes (the two points in the Earth's orbit where the Earth's tilted axis and an imaginary line drawn from the Earth to the Sun are exactly perpendicular to one another). The solstices and equinoxes divide the year up into four approximately equal parts. In the northern hemisphere winter solstice occurs on or about December 21; summer solstice is near June 21; spring equinox is around March 20; and autumnal equinox is about September 23. The effect of the Earth's axial tilt in the southern hemisphere is the opposite of that in the northern hemisphere, thus the seasons of the solstices and equinoxes in the southern hemisphere are the reverse of those in the northern hemisphere (e.g. the northern summer solstice is at the same time as the southern winter solstice).
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  6 Effects of theEarth’s Revolution: The revolution of the Earth, or its orbit around the Sun, also has several important effects: 1. Seasonal Changes: Earth's revolution around the Sun causes the varying lengths of daylight and changes in the angle of sunlight throughout the year, leading to the four seasons: spring, summer, autumn (fall), and winter. This variation in sunlight is due to the tilt of Earth's axis relative to its orbit. 2. Climate Patterns: The tilt of Earth's axis and its revolution around the Sun contribute to the distribution of solar energy across the planet, influencing global climate patterns. The varying intensity and duration of sunlight in different regions lead to temperature variations and climate zones. 3. Astronomical Events: Earth's revolution affects astronomical phenomena such as solstices and equinoxes. Solstices occur when the Sun reaches its highest or lowest point in the sky relative to the celestial equator, marking the longest and shortest days of the year. Equinoxes occur when the Sun crosses the celestial equator, resulting in nearly equal lengths of day and night. 4. Precession: Earth's revolution, combined with its axial tilt, causes a slow wobble in the orientation of its axis over time, a phenomenon known as precession. This wobble has a significant impact on the timing and orientation of the seasons over long periods of time, altering the positions of the celestial poles and affecting the alignment of the Earth's axis with respect to the stars. 5. Orbital Parameters: Earth's orbit around the Sun is not perfectly circular; it is an ellipse, with the Sun located at one of the foci. This eccentricity in the orbit leads to variations in Earth's distance from the Sun throughout the year, affecting the amount of solar radiation received by the planet. 6. Tides: Although primarily influenced by the Moon's gravitational pull, Earth's revolution around the Sun also affects tidal patterns. The alignment of the Earth, Moon, and Sun influences the strength and height of tides, particularly during spring and neap tides.
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  7 7. Astronomical Observations:Earth's revolution provides the basis for our understanding of the annual motions of celestial objects. It enables astronomers to predict the positions of planets, stars, and other celestial bodies relative to Earth at different times of the year. Overall, Earth's revolution around the Sun is fundamental to the planet's climate, seasons, and astronomical phenomena, shaping the environment and influencing life on Earth. C. Wobbling or Precessional Motion of the Earth’s Axis This motion is very similar to that of a spinning top in the Earth’s gravitational field. Besides its rotation around its own axis, the top’s axis also has a rotation around the vertical axis with a fixed frequency. This is called precessional or wobbling motion of the top. The same rule applies to the Earth. The Earth is not precisely a sphere and owing to its rotation and the fact that it is not a completely rigid, its shape has become more an oblate ellipsoid instead of a complete sphere. Indeed, the equatorial diameter of the Earth is 42 kilometers larger than the polar diameter. As a result, due to the combined tidal forces of the Sun and the moon on the Earth’s equatorial bulge, and its inclined axis of rotation relative to its orbital plane, there is a periodic motion of the Earth’s axis with a period of about 26,000 years. This has an interesting observable consequence. Although this motion is too slow to be discovered during our lifetime, yet it is observable over long periods of time. Some 5,000 years ago, the pole star was another star called Thuban (α Draconis) and not the present pole star (Polaris) that we see at nights.
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  8 D. Tilt changeof the earth’s rotation axis Although the present angle of inclination of the Earth’s axis of rotation relative to its plane of orbit around the Sun is 23.5⁰, careful observations by astronomers have made it clear that this angle is changing periodically with a period of 41,000 years from about 24.5⁰ to 22.5⁰. This motion is primarily due to the gravitational attraction of the Earth by the Sun and deviances of the Earth’s shape from a sphere. Interestingly it has been found that this movement combined with the Precessional movement of the Earth’s axis of rotation has been the major cause of the Earth’s periodic ice ages. E. Ellipticity (eccentricity) change of the Earth’s orbit around the Sun (change of eccentricity or stretch): The Earth revolves around the Sun with a period of about 365 days. The shape of the Earth’s orbit around the Sun is an ellipse with the Sun at its center. This shape indeed is not static and the ellipticity of this orbit changes over time from a complete circle to an ellipse and back. The period of this motion is not constant and it ranges from 100,000 to 120,000 years.
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  9 F. Perihelion changeof the Earth’s orbit around the Sun This motion is mainly due to gravitational forces of other planets on the Earth and leads to the regular change of the direction that the elliptical orbit of the Earth points to.
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  10 G. Change inthe orbital inclination of the Earth It has been discovered that the Earth’s plane of the orbit is not constant in time; rather, its inclination changes relative to the orbit or other planets. The average period of this motion is about 100,000 years. During this period the angle of inclination changes from 2.5⁰ to -2.5⁰.