The earth is never at rest; it is always moving. It has two main types of motion: rotation and revolution. Rotation is the spinning of the earth on its own axis, and revolution is the movement of the earth around the sun in its orbit. These two motions of the earth give us day and night, the seasons, and the cycle of years.
The earth's axis is tilted at an angle of 23.5 degrees. This tilt is responsible for many phenomena, including the change of seasons, the unequal length of day and night, and the occurrence of the longest and shortest days at particular places.
In this chapter, we will learn about rotation and revolution, the leap year, the inclined axis of the earth, and how day and night and the seasons are caused.
Rotation is the motion of the earth on its own axis. The earth spins on its imaginary axis, which passes from the North Pole to the South Pole.
The earth rotates from west to east, and it takes about 24 hours to complete one rotation. This rotation causes day and night. The part of the earth that faces the sun experiences day, and the part that faces away from the sun experiences night. The boundary between day and night on the earth is called the circle of illumination. As the earth rotates, different places come into the sunlight and go out of it, giving rise to day and night.
Revolution is the motion of the earth around the sun in a fixed path called its orbit. The earth takes about 365.25 days (one year) to complete one revolution around the sun.
The orbit of the earth is not a perfect circle; it is slightly elliptical (oval-shaped). The earth revolves around the sun, and at the same time, it also rotates on its axis. Because of the inclination of the earth's axis and the revolution of the earth, we experience different seasons, and the length of day and night changes at different places.
The earth takes about 365 days and 6 hours to complete one revolution around the sun. We count a year as 365 days, and the extra 6 hours (one-fourth of a day) are added every four years as one extra day.
This extra day is added to the month of February, which then has 29 days instead of 28 days. Such a year is called a leap year, and it has 366 days. The leap year comes every four years, for example, 2024 was a leap year.
The earth's axis is not vertical; it is tilted at an angle of 23.5 degrees from the vertical. This inclination remains the same throughout the year, and the axis always points in the same direction, towards the North Star (Pole Star).
Because of this inclination, the sun's rays do not fall equally on all parts of the earth. When the earth revolves around the sun, sometimes the Northern Hemisphere tilts towards the sun, and at other times the Southern Hemisphere tilts towards the sun. This changing tilt causes the seasons and the unequal length of day and night.
Day and night are caused by the rotation of the earth on its axis. As the earth rotates, the part facing the sun has day, and the part facing away has night.
The circle of illumination divides the earth into the light half and the dark half. Since the earth's axis is tilted, the circle of illumination does not pass through the poles. This is why, in some seasons, places near the poles have very long days or very long nights.
The revolution of the earth around the sun, combined with the inclination of its axis, causes the seasons. The earth has four main seasons: summer, winter, spring and autumn.
When the Northern Hemisphere tilts towards the sun, it receives more direct sunlight and has summer. At the same time, the Southern Hemisphere tilts away from the sun and has winter. Six months later, the situation is reversed. Between these, there are the milder seasons of spring and autumn, when the sun shines directly on the equator.
There are two important days in the yearly cycle of the earth:
On 21 June and 22 December, one pole of the earth is in complete darkness while the other pole receives sunlight for 24 hours.
An equinox is the day when day and night are equal all over the world. The word "equinox" means "equal night".
There are two equinoxes in a year: - Spring Equinox (21 March): The sun shines directly on the equator. - Autumn Equinox (23 September): The sun again shines directly on the equator.
On these days, day and night are of equal length everywhere on the earth. Between 21 March and 23 September, the Northern Hemisphere has day longer than night; between 23 September and 21 March, the Southern Hemisphere has day longer than night.
Because the earth's axis is inclined, the circle of illumination does not pass through the poles. This has important consequences:
This means that for a person standing near the North Pole, the sun does not set for months in summer and does not rise for months in winter. This phenomenon of continuous day or night is experienced in the polar regions, inside the Arctic and Antarctic circles.
| Feature | Rotation | Revolution |
|---|---|---|
| Meaning | Spinning on its own axis | Movement around the sun |
| Time taken | About 24 hours | About 365.25 days (one year) |
| Result | Day and night | Seasons and year |
| Direction | West to east | Around the sun in orbit |
| Day | Event |
|---|---|
| 21 March | Spring Equinox; day and night equal everywhere |
| 21 June | Summer Solstice; longest day in the Northern Hemisphere |
| 23 September | Autumn Equinox; day and night equal everywhere |
| 22 December | Winter Solstice; shortest day in the Northern Hemisphere |
The earth is in constant motion. Its rotation on its axis takes about 24 hours and gives us day and night, while its revolution around the sun takes about 365.25 days and gives us the year and the seasons. The tilt of the earth's axis at 23.5 degrees is responsible for the changing seasons, the solstices and the equinoxes.
On 21 June and 22 December, the two solstices mark the longest and shortest days, and on 21 March and 23 September, the equinoxes bring equal day and night. The leap year, with its extra day in February, keeps our calendar in harmony with the earth's journey around the sun. Understanding the motions of the earth helps us understand the rhythm of life on our planet, from the daily cycle of day and night to the yearly cycle of the seasons.