The Earth And its movement
The Earth
Earth is the fifth largest of the planets in the solar system, smaller than the four gas giants namely Jupiter, Saturn, Uranus and Neptune, but larger than the three other rocky planets, Mercury, Mars and Venus.
The Shape of the Earth and its Evidence
Describe the shape of the earth and its evidence
The Earth is not perfectly round. Its shape is an oblate spheroid which is a flattened sphere. The flattening is very slight at the poles than at the equator.
There are many ways to prove that the Earth is spherical. The following are some of them:
- Circumnavigation of the Earth: The first voyage around the world by Ferdinand Magellan and his crew, from 1519 to 1522, proved beyond doubt that the Earth is spherical. No traveller going round the world by land or sea has ever encountered an abrupt edge, over which he would fall. Modern air routes and ocean navigation are based on the assumption that the Earth is round. If you travel in an aeroplane in a straight path, flying non-stop, you will eventually come back where you started your journey. This is what is called circumnavigation.
- The circular horizon: The distant horizon viewed from the deck of a ship at sea, or from a cliff on land is always and everywhere circular in shape. This circular horizon widens with increasing altitude and could only be seen on a spherical body.
- Ship’s visibility: When a ship appears over the distant horizon, the top of the mast is seen first before the hull. In the same way, when it leaves harbour, its disappearance over the curved surface is equally gradual. If the Earth were flat, the entire ship would be seen or obscured all at once.
- Sunrise and sunset: The sun rises and sets at different times in different places. As the Earth rotates from west to east, places in the east see the sun earlier than those in the west. If the Earth were flat, the whole world would have sunrise and sunset at the same time. But we know this is not so.
- The lunar eclipse: The shadow cast by the Earth on the moon during a lunar eclipse is always circular. It takes the outline of an arc of a circle. Only a sphere can cast such a circular shadow.
- Planetary bodies are spherical: All observations from telescopes reveal that the planetary bodies, the sun, moon, satellites and stars have circular outlines from whichever angle you see them. They are strictly spheres. Earth, by analogy, cannot be the only exception.
- Driving poles on level ground on the Earth: Engineers when driving poles of equal length at regular intervals on the ground have found they do not give a perfect horizontal level. The centre pole normally projects slightly above the poles at either end because of the curvature of the Earth. Surveyors and field engineers therefore have to make certain corrections for this inevitable curvature, i.e. 12.6 cm to 1 km.
- Space photographs: Pictures taken from high altitudes by rockets and satellites show clearly the curved edge of the Earth. This is perhaps the most convincing and the most up-to-date proof of the Earth's sphericity.
The Earth and all other planets revolve around the sun.
Types of Earth's Movements
Describe the types of earth's movements
The Earth is in motion all the time. People cannot feel this motion because, like all other planets, they move with it. There are two types of movements of the Earth, namely:
- The rotation of the Earth on its own axis.
- The revolution of the Earth around the sun.
The Term Rotation
Describe the term rotation
Rotation refers to the spinning of a body on its axis. The Earth rotates or spins on its axis in an anti-clockwise direction, from West to East through 360 degrees. It makes one complete rotation in 24 hours. Thus, for every 15 degrees of rotation, the Earth takes one hour which is the same as four minutes for every 1 degree.
An axis is an imaginary line joining the N (North) and S (South) poles through the centre of the Earth.
The rotation of the Earth is very rapid although it is difficult to feel its motion. At the equator, every point of the Earth's surface is travelling eastwards at about 1600 km per hour. At latitude 40 degrees, the speed is about 1280 km per hr.
The Earth’s axis makes an angle of 66 ½ degrees with the plane of its orbit. In other words, the axis is tilted 23 ½ degrees from the perpendicular.
Fig. 1.11 Rotation of the Earth
Evidence to Prove that the Earth Rotates
Give evidence to prove that the earth rotates
The following observations illustrate that the Earth rotates from West to East:
- When travelling in a fast moving vehicle, you notice that trees and other objects on both sides of the road are moving fast in the opposite direction. This observation is similar to the movement of the Earth in relation to the sun.
- At night most of the stars appear to move across the sky from West to East. This observation shows that the point of observation (Earth) is moving from West to East.
- Sunrise and sunset: the sun rises over the eastern horizon in the morning and sets over the western horizon in the evening. But since the sun is in the centre of the solar system and the fact that it does not move, this shows that the point of observation (the Earth) is moving by rotation from West to East.
- Day and Night: During the Earth's rotations some regions face the sun while others do not face it. The regions facing the sun experience day time whereas the regions which are not facing the sun are in darkness (night). If the Earth was not rotating, one half of the Earth would be having daylight while the other half would be in total darkness forever. The occurrence of day and night proves that the Earth is rotating.
- Photographs of the Earth taken from the satellite at different times of the day show that different parts of the Earth experience daylight at different times. If the Earth was not rotating, different photographs taken at any time of the day would all look alike.
Fig. 1.12 Day and night
Significances of Earth's Rotation
Explain the significances of earth's rotation
The rotation of the Earth is very important because it causes the following phenomena:
- Alternation of day and night: Rotation of the Earth causes the side of the Earth which face the sun to experience daylight which is the day, whereas the side that is not facing the sun at that time will be in darkness (night). This ensures that, at any time of the day, one half of the Earth is in darkness and the other is in daylight. If the Earth did not rotate then its one half would be in daylight while the other half would be in darkness all the year round.
- The occurrence of tides: Tides are the periodic rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun on the rotating Earth.
- Deflection of winds and ocean currents: As the Earth rotates on its axis from West to East, winds and ocean currents flowing over the Earth’s surface are deflected. The anticlockwise rotation of the Earth deflects prevailing winds to the right in the northern hemisphere and to the left in the southern hemisphere.
- Time difference between longitudes: The rotation is responsible for the difference in time between different places on Earth. It causes the difference of one hour in every 15 degree interval between longitudes, which is equivalent to 4 minutes for each degree of longitude.
Fig 1.13 Deflection of winds and ocean currents
The Term Revolution
Define the term revolution
In Geography and Astronomy, the word rotation is defined as the motion of one body around another. The Earth revolves around the sun and the moon revolves around the Earth.
The Process of Revolution
Explain the process of revolution
The Earth is at aphelion each year on 4th July, when it is at the maximum distance of 152 million kilometres form the sun. The Earth is at perihelion each year on 3rd January when it is at the minimum distance of 147 million kilometres.
The Earth's revolution around the sun takes a year (365¼ days). Therefore, the average speed of revolution is about 29.6 kilometres per second. A normal year has only 365 days. The remaining fraction of ¼ day is added once in four years to make a leap year of 366 days.
Fig 1.14 Revolution of the Earth
The Result of the Earth's Revolution Around the Sun
Describe the result of the earth's revolution around the sun
The revolution of the Earth around the sun and the inclination (tilting) of its axis results in the following:
- The four seasons of the year, namely summer, autumn, winter and spring. A season is one of the distinct periods into which the year may be divided.
- Change in the altitude of the midday sun at different times of the year at any place.
- Varying lengths of the day and night at different times of the year. The axis of the Earth is inclined to its elliptical plane at an angle of 66.5 degrees. If the axis of the Earth was vertical, the sun rays would always be overhead at the Equator, thus all places on the Earth would always experience 12 hours of daylight (day) and 12 hours of darkness (night).
- The eclipses (eclipse of the sun and eclipse of the moon).
Four seasons
Because of the inclination of the Earth’s axis to the orbital plane, the angle at which the sun rays shrike the Earth’s surface varies. This leads to seasonal changes which are mainly experienced in the high and mid latitude regions. This results in four distinct seasons namely, spring, summer, autumn and winter.
Fig 1.15 The seasons
Seasons are not experienced in the equatorial regions (for example most parts of Africa, Tanzania inclusive) because the sun is almost overhead at all places and the lengths of days and nights are almost equal throughout the year.
Change in altitude of the midday sun
Because of the inclination of the Earths’ axis, the midday sun is directly overhead at the Tropic of Cancer on 21st June; and at the Tropic of Capricorn on 22nd December. This is called the solstice. On 21st June, it is winter solstice in the southern hemisphere and summer solstice in the northern hemisphere. On 22nd December, it is winter solstice in the northern hemisphere and summer solstice in the southern hemisphere. On 21st march and 23rd September, the midday sun is directly overhead at the Equator. These are the only two days in the year when all places on Earth have almost equal hours of day and night. This is known as the equinox.
Table 1.2 shows dates on which the sun is vertically overhead at the Tropic of Cancer, Tropic of Capricorn and the Equator. The resulting seasons are also indicated.
Table 1.2 dates of overhead sun in different parallels of latitude
| Latitude | Date of overhead sun | Hemisphere | Name |
| Equator | 21st March | Northern | Spring equinox |
| Southern | Autumnal equinox | ||
| 23rd September | Northern | Autumnal equinox | |
| Southern | Spring equinox | ||
| Tropic of Cancer | 21st June | Northern | Summer solstice |
| Southern | Winter solstice | ||
| Tropic of Capricorn | 22nd December | Northern | Winter solstice |
| Southern | Summer solstice |
Varying lengths of day and night
Not all places across the Earth experience the same lengths of day and night. Some places receive long hours of daylight than darkness while others receive long hours of darkness than daylight. This is because the Earth’s axis is inclined at 66 ½ degrees to the orbital plane (23 ½ degrees to the perpendicular). The Earth remains permanently inclined at this angle as it revolves around the sun.
If the Earth’s axis was perpendicular to its orbital plane, all places on the Earth’s surface would have equal days of daylight and darkness throughout the year.
In December, it is winter in the southern hemisphere and the hours of darkness increase steadily. The further away a place is from the equator, the longer the nights. Beyond the Arctic Circle (66 ½ºN) towards the North Pole, the number of days of complete darkness increase. The North Pole is in complete darkness for half a year.
Eclipses
Eclipse refers to the partial or complete obscuring of one celestial (heavenly) body by another. An eclipse occurs when one celestial body moves in between another heavenly body and its source of light (the sun). Eclipses normally occur when the sun or moon is obscured from the view for a short period. This means that an eclipse will only occur when the sun, moon and Earth are in straight or nearly straight line.
Now, how are eclipses related to revolution of the Earth? The answer is simple: as the Earth revolves around the sun and the moon revolves around the Earth, there comes a time when the moon and the Earth are in a straight line. As a result, an eclipse of the moon (lunar eclipse) or the sun (solar eclipse) occurs, depending on which body (the Earth or the moon) causes an obstruction:
Lunar eclipse
This eclipse occurs when the Earth passes between the moon and the sun and the Earth’s shadow falls on the moon.
Fig 1.16 Lunar eclipse
Solar eclipse
Solar eclipse occurs when the Moon passes between the Earth and the Sun, thus casting its shadow onto the Earth. In this type of eclipse the Moon fully or partially blocks the Sun, leading to total or partial eclipse respectively.
Fig 1.17 Solar eclipse
Subscribe to:
Post Comments
(
Atom
)
No comments :
Post a Comment