EARTH HAS TWO primary motions: revolution and rotation. The first refers to the earth’s annual orbit of the sun, which takes a bit more than 365 days per year (hence, a leap year with 366 days every 4 years in order to “catch up”). As it revolves around the sun, the Earth rotates on its axis once every 24 hours, a period of time referred to as a mean solar day or sol. The axis of rotation is an imaginary line that passes through both the geographical North and South Poles. It is important to note that the Earth’s axis is tilted approximately 23.5 degrees from the ecliptic, defined as an imaginary plane described by the revolution of the earth around the sun. The direction of the Earth’s rotation may be determined by viewing the Earth from a point in space above either pole. An observer above the North Pole would note a counterclockwise motion of the Earth to the east. An observer would see a similar eastward motion over the South Pole, but from this perspective the motion would be clockwise. The eastward rotation of the Earth accounts for the apparent westward motion of the celestial bodies (sun, moon, stars) throughout the 24-hour period. An observer in the Northern Hemisphere on a clear night can gain evidence of rotation by noting the apparent east to west movement of stars in the vicinity of Polaris, which aligns well with the Earth’s axis and remains in a stationary location. The speed of the Earth’s rotation varies depending on the latitudinal position of the observer. The speed is greatest at the equator where the circumference of the Earth is at a maximum. The general rule for determining the speed of the Earth at any latitude is straightforward:
speed = circumference of latitude/24
The circumference of the earth at the equator is approximately 25,000 mile (40,000 kilometer). Applying the expression using these inputs results in a speed of 1,150 mile per hour (1,700 kilometer per hour). At the latitudes of 90 degrees north and 90 degrees south (the poles) the speed of rotation is zero. Latitudes between the poles and the equator will have rotational speeds more than zero and less than that of the equator. Rotation should not be interpreted as a spinning motion. The Earth rotates, and even though this motion has similarities to the spinning motion of a top or a figure skater, it is not the same. Spinning implies a rapid whirling motion not linked to a specified axis. Both the top and the figure skater spin in association with a wavering axis of motion. The Earth’s rotation, on the other hand, is regular and invariably related to its clearly specified, observable, and measurable axis. The motions of revolution and rotation, both universally accepted now, were not understood until well into the 16th century. For centuries, the Ptolemaic system held that the Earth was the center of the universe and all celestial bodies revolved around it.
This theory held sway in the scientific word from the time of Ptolemy throughout the Middle Ages until it was proven false in the 16th and 17th centuries. The first to proclaim that the Earth and the other planets revolved around the sun was Nicolas Copernicus, a Polish astronomer who published his theory in 1543, the year of his death. Copernicus also claimed that the Earth rotated on its axis. Additional support for Copernicus came from Johannes Kepler, a German astronomer who rejected Ptolemy’s concept of circular revolution and proposed the idea of the elliptical motion of the planets. Finally, it was Galileo who demonstrated the accuracy of the Copernican theory and developed a comprehensive mathematical proof of the heliocentric system. The Earth’s rotation produces a constantly changing diurnal (daily) system of daylight and darkness, which is sensed and responded to by plants and animals alike. Rotation also produces changes in the amount of heat accumulated and lost during the 24- hour diurnal cycle. Another interesting result of rotation is the diversion of air masses in the atmosphere in predictable directions, phenomena known as Coriolis force. Due to the Earth’s rotation, high-pressure air masses in the Northern Hemisphere will be diverted in a clockwise direction, whereas low-pressure air will divert in a counterclockwise direction. These directional diversions are reversed in the Southern Hemisphere.
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