The earth has two types of motions, namely rotation and revolution. Rotation is the movement of the earth on its axis. The movement of the earth around the sun in a fixed path or orbit is called Revolution.
- Imagine a line passing through the center of Earth that goes through both the North Pole and the South Pole. This imaginary line is called an axis.
- Earth spins around its axis, just as a top spins around its spindle. This spinning movement is called Earth’s rotation.
- The axis of the earth which is an imaginary line, makes an angle of 66½° with its orbital plane.
- The plane formed by the orbit is known as the orbital plane.
- The earth receives light from the sun.
- Due to the spherical shape of the earth, only half of it gets light from the sun at a time.
- The portion facing the sun experiences day while the other half away from the sun experiences night.
- The circle that divides the day from night on the globe is called the circle of illumination. This circle does not coincide with the axis.
- The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earthday. This is the daily motion of the earth.
The earth rotates on its axis taking approximately 24 hours to complete one rotation. This has important environmental consequences.
- Rotation creates a diurnal cycle of light and darkness, temperature, and humidity changes.
- Rotation causes the day-night cycle which also creates a corresponding cycle of temperature and humidity creates a corresponding cycle of temperature and humidity
- 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 complicated because because they are the result of both the gravity 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.
- At the same time that the Earth spins on its axis, it also orbits, or revolves around the Sun. This movement is called revolution.
- It takes 365¼ days (one year) to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for the sake of convenience.
- Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February.
- Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.
- The gravitational pull of the Sun keeps Earth and the other planets in orbit around the star. Like the other planets, Earth’s orbital path is an ellipse so the planet is sometimes farther away from the Sun than at other times.
- The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd and the furthest is at aphelion (152 million km) on July 4th.
- Earth’s elliptical orbit has nothing to do with Earth’s seasons.
- During one revolution around the Sun, Earth travels at an average distance of about 150 million km.
- Earth revolves around the Sun at an average speed of about 27 km (17 mi) per second, but the speed is not constant.
- The planet moves slower when it is at aphelion and faster when it is at perihelion.
- The reason the Earth (or any planet) has seasons is that Earth is tilted 23 1/2oon its axis.
- During the Northern Hemisphere summer the North Pole points toward the Sun, and in the Northern Hemisphere winter the North Pole is tilted away from the Sun.
- 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 in the Southern Hemisphere all these conditions are reversed.
- It is winter season there.
- The nights are longer than the days.
- This position of the earth is called the Summer Solstice
- 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.
- The reverse happens in the Northern Hemisphere.
- This position of the earth is called the Winter Solstice
- 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.
- On 23rd September, it is autumn season in the Northern Hemisphere and spring season in the Southern Hemisphere.
- The opposite is the case on 21st March, when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
Effects of revolution
- Revolution along with the earth’s tilted axis leads to changing seasons across the hemispheres.
- The speed of the Earth’s revolution has influenced the state of the Earth. On account of the speed of pivot, a diffusive power is made which prompts the straightening of the Earth at shafts and protruding at the middle.
- The Earth’s revolution influences the development of water in the seas. The tides are redirected because of the turn.
- The speed of revolution additionally influences the development of the breeze. Because of revolution, winds and the sea flow redirect to one side in the Northern Hemisphere and to one side in the Southern Hemisphere.