Earth is just the right distance from the Sun for its temperature to let water exist as a liquid, a solid, and a gas. The presence of water as a liquid on Earth’s surface throughout its long history also enabled life to evolve. Another characteristic of Earth is that it is dynamic. Its interior and surface continually change as a result of its internal heat.
The axis of the Earth is tilted at an angle of 23 degrees from the vertical. That is, the earth’s axis forms an angle of 23½ degree with the perpendicular to the plane of the earth’s orbit. This means that the earth’s axis makes an angle of 66½ o with the orbital plane. Therefore, the earth is always inclined towards one side while revolving around the sun. The tilting of the Earth’s axis is also called the inclination of earth’s axis. The spherical shape of the earth results in only half of the Earth facing the sun getting the sunlight while, the other half is in darkness.
There are three basic movements of the Earth: (i) Galactic movement, (ii) Rotation, and (iii) Revolution.
1. Galactic Movement
This is the movement of the Earth with the sun and the rest of the solar system in an orbit around the center of the Milky Way Galaxy. This movement has the little effect in changing the environment of the Earth.
The first primary motion of the earth is rotation. The earth rotates in a counterclockwise pattern, spinning on its axis once every 24 hours. The earth wobbles as it rotates, similar to a spinning top because the rotation of the earth occurs at a slight tilt. The earth is tilted at approximately 23.5 degrees. The tilt of Earth, as it rotates, creates different gravitational forces at the north and south poles. The Earth rotates counterclockwise; this is why the Sun “rises” in the East and “sets” in the West. It is not the Sun’s movement that causes days, but rather the Earth turning around in front of the Sun.
This has important environmental consequences.
- Rotation creates a diurnal cycle of light and darkness, temperature, and humidity changes.
- Rotation requires the creation of standardized time zones. There are 24, one for each hour of the earth’s rotation.
- Rotation causes the tides‐ the twice daily rise and fall of sea level. Tides are complicated because they are the result of both the gravity of the moon and the gravity of the sun. Sometimes the sun and the moon are lined up with the earth, but most of the time they are not. Tides are highest when the earth, sun and moon are in a straight line.
- The Coriolis Force. Rotation causes a deflection of ocean and air currents. The earth rotates much faster than the winds or currents move. This causes a large deflection in the direction that winds move and ultimately results in rotation around low pressure cells and high pressure cells. It also causes large rotating pools of water in the oceans called gyres. The Coriolis force only operates on large features.
- At the Equator the eastward velocity is 1700 Km/hr; at the poles it is 0
- A difference of 1 hour between two meridians which are 15 degrees apart.
- Earth rotates at a rate of 15 degrees per hour. To complete one rotation the Earth has to spin around 360 degrees.
Earth is currently in a cool phase characterized by formation of glaciers (glacial maxima), followed by warm periods with glacial melting (interglacial periods). These glacial–interglacial cycles occur at frequencies of about 100,000 years. We are currently in an interglacial period; these have lasted about 23,000 years in the past. The last glacial maximum was about 18,000 years ago.
The glacial–interglacial cycles have been explained by regular changes in the shape of Earth’s orbit and the tilt of its axis—Milankovitch cycles.
Circular rotation causes glaciers to melt; more solar radiation; Elliptical= less radiation. The intensity of solar radiation reaching Earth changes, resulting in climatic change. The shape of Earth’s orbit changes in 100,000-year cycles. The angle of axis tilt changes in cycles of about 41,000 years. Earth’s orientation relative to other celestial objects changes in cycles of about 22,000 years.
The inner core of Earth rotates faster than the crust. Earth is made up of an inner core, an outer core, the mantle and the crust. The inner core, a solid mass of iron about 1,500 mi (2,400 km) in diameter, is suspended in the molten metal of the outer core, which is about 1,400 mi (2,240 km) thick. The mantle of the earth is a layer of mostly solid material about 1,700 mi (2,720 km) thick; the crust itself varies from 4 to 25 mi (6 to 40 km) thick. Although the inner core spins in the same direction as Earth’s crust, the inner core rotates at a different speed. Scientists estimate that the inner core of Earth spins from one to three degrees a year faster than the crust of Earth. This means that while a point on the crust of Earth moves 360° in a year, a similar point on the inner core would move 361-363° in a year. Thus, over approximately 360 years, the inner core would make one complete rotation more than the crust of Earth.
The Earth revolves around the Sun once every 365.242199 mean solar days. The Earth orbits the Sun at a speed of 108,000 km/h. Earth’s perihelion(147,098,074 km) occurs around January 3, and the aphelion around July 4 (152,097,701 km) .
The Sun rotates on its axis in an anticlockwise direction. The Earth also rotates on its axis in an anticlockwise direction. And the Earth revolves around the Sun in an anticlockwise direction. All the other major planets, and most of the minor planets (asteroids) also orbit the Sun in an anticlockwise direction. (A few comets orbit in the opposite, or clockwise, direction).
Most of the planets rotate about their own axes in an anticlockwise direction. This is called prograde rotation. Venus and Uranus are exceptions. Venus has a very slow clockwise rotation (termed retrograde), and Uranus, although it rotates every 15.5 hours, has its equator inclined at almost a right angle to its orbital plane. It is thus difficult to know whether to describe the rotation of Uranus as pro-grade with an equatorial inclination of 98o or retrograde with an inclination of 82o.
The motion around the sun along its orbit is called a revolution. The amount of time it takes for a single trip around the sun is called a period of revolution. The period for the Earth to revolve around the sun is 365.24 days or one year. The .24 days is why every four years February has 29 days. Earth’s orbit is not a perfect circle. In other words, It takes the earth 365¼ days , that is, a year for one complete revolution, the ¼ or 6 hours is added up in every four years to make a leap year. Instead Earth’s orbit is slightly elliptical in shape.
The position of the earth changes around the sun as follows:
- On 21st June, the Northern Hemisphere is tilted towards the sun. The rays of the sun fall directly on the Tropic of Cancer. As a result, these areas receive more heat. The areas near the poles receive less heat as the rays of the sun are slanting. The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months. Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21st June. At this time winter season occurs in the Southern Hemisphere.
- On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23½° S), a larger portion of the Southern Hemisphere gets light. Therefore, it is summer in the Southern Hemisphere with longer days and shorter nights.
- On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.
Effect Of Revolution Of The Earth
1. Varying length of day and night : due to the inclination of the earth axis on the plane of elliptical.
Plane of elliptical is the plane which the earth orbits round the sun.
The earth is inclined at angle which varies the length of day and night.
In the northern hemisphere, winter (December), as we progress northward the hours of darkness gradually increases, at the arctic circle 66½N the sun never rises which causes the total darkness for the whole day. Whereas in the southern hemisphere, the period of daylight increases towards the south pole in December 22nd but the region experience summer.
In the summer (june), daylight increases in the northern hemisphere as we move towards the north pole, at the arctic circle, the sun never sets at mid summer (june) and there is a complete 24hours period of continuous daylight.
During summer, the region of arctic circle is popularly referred to as land of midnight sun.
In the southern hemisphere, the period of darkness increases towards the south pole in June 21st but the region experience winter.
2. It causes season : In tropical areas like west Africa, there are two seasons, the rainy and the dry season, for example in Nigeria, the is only two seasons, it is either raining or dry season
But in temperature countries, like USA, the revolution of the earth causes four seasons, winter, spring, autumn and summer.
3. it determines a year : it is the time taken by the earth to move round the sun, that is, to complete it orbit that determines a year, it could take the earth 365 days or 365¼ or plus 6 hours to complete its orbit.
4. Changes the altitude midday of the sun : due to the revolution of the earth round the sun with its axis inclined at 66½ to the plane of elliptical changes the apparent altitude of the midday sun.
The sun is vertically overhead at the equator on two days each year, these two days are 21st on march ( spring equinox) and 23rd of September ( autumn equinox), equinox means equal nights because on these two days all part of the world have equal day and night.
After march 21, the sun appears to move towards north and is vertically overhead in the tropical cancer at an inclination angle of 23½°N on about june 21, this is known as the june or summer solstice when the northern hemisphere have longest day and shortest night.
Later in december 22nd the sun appears to move southward and vertically overhead at the tropical capricorn at an inclination angle of 23½°S, this is known as december or winter solstice when the southern hemisphere have its longest day and shortest night.
The tropic thus mark the limit of overhead, for beyond these, the sun is never overhead at anytime of the year, such regions are marked by distinct season changes, which are spring, winter, autumn and summer.
5. Effect on temperature : seasonal changes thus have effect on temperature, summer is usually associated with much heat and daylight and winter with cold and darkness.
This happens because the sun is higher in the sky than in winter.
When the sun is overhead, its rays is almost vertically on the earth, concentrating its heat on small area, therefore temperature rises and summer are always warm.
During winter, the oblique(slant) rays of the sun coming through the atmosphere lies directly and have much of the heat been absorbed by the atmosphere impurities and water vapor, The sun’s ray fall faintly and spread over a great area.