NCERT / CBSE NOTES : Chapter Summary
Motions of the Earth
The movement of the earth spinning around itself on its axis is called rotation. The imaginary line around which the earth spins or rotates is called the axis of the earth. The earth’s axis is at an angle of 66 ½ to the orbital plane, which is the plane formed by the earth’s orbit.
If a vertical line is drawn perpendicular to the orbital plane, the axis forms an angle of 23 ½. This is called the earth’s angle of inclination.
The earth takes 24 hours to complete one rotation around itself. While the earth rotates, the sun remains stationary.
As the earth rotates, only half of it faces the sun at any time, while the other half faces away from the sun. It is daytime in the part that faces the sun, while it is night in the other half.
The circle that divides the day and night on the globe is called the circle of illumination. The earth spins from west to east. One side of the earth would always face the sun and would get extremely hot, while the other side would always have night and would become too cold. Thus, night and day are caused because of the earth’s rotation.
A year is 365 days long, because that’s about how long it takes for the earth to complete one full revolution around the sun. The earth rotates or spins around its own axis. It revolves around the sun in an elliptical orbit. One full revolution around the sun takes exactly 365 ¼ days.
Every fourth year is a leap year - the extra day in a leap year results by adding the 1/4th days or 6 hours each year over 4 years.
The earth’s axis is inclined at an angle of 23-½ degrees to the vertical. This tilt and the revolution is the reason for different seasons.
The northern hemisphere receives direct sunlight, for the longest duration on the June 21st. This position of the earth is called the Summer Solstice. In summers, the days are longer and the nights are shorter. The areas beyond the Arctic Circle have daylight for 6 months at a stretch.
The reverse happens in the southern hemisphere; with less sunlight, the days are shorter and the nights are longer. The northern hemisphere receives sunlight for the shortest duration December 22. This position of the earth is known as the Winter Solstice.
During its orbit, there are times when neither of the earth’s poles is inclined towards the sun. Thus, on two occasions - March 21 and September 23, sunlight falls directly on the equator. This position of the earth is called an Equinox. On these days, the day and night are of equal duration on earth.
On September 23, it is autumn season in the Northern Hemisphere and spring in the Southern Hemisphere. The opposite is the case on March 21, when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.