Revolution, Inclination of the Axis and Seasons
Geography › Earth, Latitudes, Longitudes and Motions · Topic 1.4
Revolution is the movement of the Earth around the Sun in an elliptical orbit in about 365 days 6 hours. Because the axis is tilted at 23.5 degrees to the normal and keeps its direction in space, the Sun's rays strike different latitudes at different angles through the year, which produces the seasons, the changing length of day and night, and perihelion and aphelion.
5 min read · UPSC Prelims · Geography · Earth, Latitudes, Longitudes and Motions
At a Glance
- Motion Earth around the Sun, west to east
- Period 365 days 6 hours
- Orbit Elliptical
- Axial tilt 23.5 degrees to the normal
- Main effect Seasons and changing day length
- Exam link UPSC Prelims, Geography: Earth, Latitudes, Longitudes and Motions
Where Does It Fit?
This is the fourth topic of the category on the Earth, latitudes, longitudes and motions. The previous topic covered rotation, and this one covers the yearly movement around the Sun and the seasons. The next topic covers solstices, equinoxes and day length. Previous topic: Rotation of the Earth and Its Effects. Next topic: Solstices, Equinoxes and Variation in Day Length.
What Is the Revolution of the Earth?
In short: revolution is the yearly journey of the Earth around the Sun.
The movement of the Earth in its orbit around the Sun is called revolution. The Earth revolves in an elliptical orbit in the same direction as its rotation, from west to east, and takes 365 days and about 6 hours to complete one revolution. This period is called a year. Its average orbital speed is about 30 km per second. Since the orbit is an ellipse and not a circle, the distance between the Earth and the Sun changes during the year. Rotation, which gives day and night, is described in Rotation of the Earth and Its Effects.
What Are Perihelion and Aphelion?
The Earth is at its closest to the Sun on about 3 January, a position called perihelion, when the distance is about 147 million km. It is at its farthest on about 4 July, a position called aphelion, when the distance is about 152 million km. The average distance is about 150 million km. The distance does not cause the seasons, because the northern hemisphere has summer when the Earth is at aphelion; the seasons come from the tilt of the axis.
| Position | Date (about) | Distance from the Sun |
|---|---|---|
| Perihelion | 3 January | About 147 million km |
| Aphelion | 4 July | About 152 million km |
| Average | Through the year | About 150 million km |
Why Do We Have a Leap Year?
The Earth takes about 365 days and 6 hours to revolve, while the calendar year has 365 days. The extra 6 hours of every year add up to one full day in four years. To correct the calendar, one day is added to February every fourth year, giving a leap year of 366 days. Years divisible by 4 are leap years, but century years are leap years only if they are divisible by 400, so 2000 was a leap year and 1900 was not.
What Is the Inclination of the Axis?
The axis of the Earth is not perpendicular to the plane of its orbit. It makes an angle of 66.5 degrees with the orbital plane, or 23.5 degrees with the normal to it. In addition, during the revolution the axis keeps pointing in the same direction in space, towards the Pole Star. This is called the parallelism of the axis. These two facts together, the tilt and the constant direction, cause the seasons. If the axis were upright, every place would get equal day and night throughout the year and there would be no seasons.
How Are Seasons Caused?
As the Earth revolves with its tilted axis fixed in direction, first the northern hemisphere and then the southern hemisphere is turned towards the Sun. The hemisphere tilted towards the Sun receives more direct rays and longer days, which is summer, while the other hemisphere receives slanting rays and shorter days, which is winter. Spring and autumn are the transitions in between, when neither hemisphere is tilted strongly towards the Sun. Seasons in the two hemispheres are therefore opposite: when it is summer in India it is winter in Australia.
| Period (northern hemisphere) | Season | Position |
|---|---|---|
| 21 March to 21 June | Spring into summer | Sun moves from equator to Tropic of Cancer |
| 21 June to 23 September | Summer into autumn | Sun moves back from the Tropic of Cancer |
| 23 September to 22 December | Autumn into winter | Sun moves to the Tropic of Capricorn |
| 22 December to 21 March | Winter into spring | Sun returns towards the equator |
The detailed dates and the extra effect on the length of day are in Solstices, Equinoxes and Variation in Day Length. As an analytical point, the Indian monsoon calendar of four seasons is a regional pattern that is shaped by the same tilt but carries its own names; the astronomical seasons described here are the global model.
Why Is This Topic Important?
- Seasons Core concept in Prelims and Mains
- Perihelion and aphelion Frequent factual question
- Axial tilt Basis of day length and solstices
What Came Next?
The next topic describes the solstices, the equinoxes and the variation in the length of day. Next topic: Solstices, Equinoxes and Variation in Day Length.
Key People and Terms
- Revolution Movement around the Sun
- Perihelion Earth closest to the Sun
- Aphelion Earth farthest from the Sun
- Leap year Year of 366 days
- Parallelism of the axis Axis keeps its direction in space
- Orbital plane Plane of the Earth’s orbit
Exam Corner
Points to Remember
- Revolution takes 365 days 6 hours.
- Perihelion is on about 3 January; aphelion on about 4 July.
- The axis is tilted 23.5 degrees to the normal.
- Seasons are caused by the tilt, not by the distance from the Sun.
- A century year is a leap year only if divisible by 400.
Do Not Confuse With
- Perihelion and aphelion Nearest versus farthest
- Tilt and distance as cause of seasons Tilt is the cause
- 66.5 degrees and 23.5 degrees Angle with the orbital plane versus with the normal
Memory Hook
Tilt makes the seasons, not distance; January near, July far, 23 and a half is the lean.
Mains Angle
Questions ask how seasons arise. These points add depth.
- Cause Tilt plus constant direction of the axis
- Hemispheres Opposite seasons
- Distance Not the cause
A Question You May Face
An original practice question, not a past paper question.
“Explain how the inclination of the Earth’s axis and its revolution cause the seasons.”
How to Answer
- Define Revolution and tilt.
- Mechanism Parallelism of the axis.
- Result Angle of rays and day length.
- Clarify Distance is not the cause.
GS Relevance
Frequently Asked Questions
What is the revolution of the Earth?
Revolution is the movement of the Earth around the Sun in an elliptical orbit, from west to east. It takes 365 days and about 6 hours to complete, and its average orbital speed is about 30 km per second.
What are perihelion and aphelion?
Perihelion is the position of the Earth nearest to the Sun, about 3 January, at a distance of about 147 million km. Aphelion is the farthest position, about 4 July, at about 152 million km. These do not cause the seasons.
Why is there a leap year?
The Earth takes about 365 days and 6 hours to revolve, so the extra 6 hours add up to one day in four years. To correct the calendar, one day is added to February every fourth year, giving a leap year of 366 days.
What is the parallelism of the Earth's axis?
The axis of the Earth is tilted at 23.5 degrees to the normal to the orbital plane, and during the revolution it keeps pointing in the same direction in space, towards the Pole Star. This constant direction, with the tilt, causes the seasons.
What causes the seasons on the Earth?
The seasons are caused by the inclination of the axis together with the revolution. The hemisphere tilted towards the Sun gets more direct rays and longer days, which is summer, while the other gets slanting rays and shorter days, which is winter.
PYQ Practice — Statement Analysis
1 The Earth is nearest to the Sun at perihelion, about 3 January.
The distance is about 147 million km.
2 The seasons are caused mainly by the changing distance of the Earth from the Sun.
They are caused by the tilt of the axis.
3 The northern hemisphere has summer when the Earth is at aphelion.
Aphelion is about 4 July.
4 The year 1900 was a leap year.
A century year must be divisible by 400.
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