Rotation is the
movement of the earth on its axis.
The axis of the earth which is an imaginary line,
makes an angle of 66° with its orbital plane. The
plane formed by the orbit is known as the orbital plane.
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 earth day. This is
the daily motion of the earth.
The second motion of the earth around the sun in
its orbit is called revolution. It takes 365 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 earth is
going around the sun in an elliptical orbit.
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 (23 ° 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.