Earth: Motions of the Earth

Rotation

Earth rotates along its axis from west to east.
It takes approximately 24 hrs to complete one rotation.
Earth rotates in a counter-clockwise direction.
Days and nights occur due to rotation of the earth.
The circle that divides the day from night on the globe is called the circle of illumination.
Earth rotates on a tilted axis. Earth’s rotational axis makes an angle of 23.5° with the normal.
It makes an angle of 66.5° with the orbital plane.
Effects
- Rotation creates a diurnal cycle of light and darkness, temperature, and humidity changes.
- Rotation requires the creation of standardised time zones. There are 24, one for each hour of the earth’s rotation.
- Rotation causes the tides.
- Rotation causes a deflection of ocean and air currents.

Revolution

The second motion of the earth around the sun in its orbit is called revolution.
It takes 365¼ days (one year) to revolve around the sun.
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 has 29 days instead of 28 days. Such a year with 366 days is called a leap year.
The closest Earth gets to the Sun each year is at perihelion (147 million km) around January 3rd and the furthest is at aphelion (152 million km) on July 4th.
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.
Effects
- Occurrence of various seasons (spring, summer, autumn and winter).

Solstices

Summer solstice
- 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 here.
- 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 southern hemisphere experiences winter and nights are longer than the days.
Winter solstics
- 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.
Equinox
- 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.