Maarg Manthan · Topic 1.9

Inclination of the Earth’s Axis and Its Effects

Geography › Earth's Origin, Interior and Motions · Topic 1.9

The Earth's axis is tilted at about 23.5° from the perpendicular to the plane of its orbit around the Sun, or 66.5° to the orbital plane itself. The axis keeps pointing in the same direction in space, called parallelism of the axis, as the Earth revolves. Because of this tilt, the Sun's vertical rays shift between the Tropic of Cancer and the Tropic of Capricorn, which causes the seasons, the changing length of day and night, the heat zones and the polar day and night.

6 min read · UPSC Prelims · Geography · Earth's Origin, Interior and Motions

At a Glance

Tilt 23.5° from the normal; 66.5° from the orbital plane
Axis direction Fixed in space (parallelism)
Main effect Seasons
Other effects Day length, polar day and night, heat zones
Long-term change Tilt varies between 22.1° and 24.5° every 41,000 years
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic follows the one on rotation and revolution and explains the single feature that turns the yearly revolution into seasons. It leads to the topic on solstices and equinoxes, which fixes the dates when the effects of the tilt are at their extremes. Previous topic: Motions of the Earth: Rotation and Revolution and Their Effects. Next topic: Local and Standard Time, International Date Line, Calendar.

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 Equinoxes and Solstices (Summer Solstice, Winter Solstice, Vernal and Autumnal Equinox). 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.

What Are the Effects of the Inclination of the Axis?

Effect	How the tilt causes it
Seasons	The hemisphere tilted towards the Sun gets more direct rays and longer days, and has summer
Shift of the vertical Sun	The overhead Sun moves between 23.5° N and 23.5° S in a year
Tropics and Polar Circles	Their latitudes of 23.5° and 66.5° come directly from the angle of tilt
Heat zones	The torrid, temperate and frigid zones are marked by these lines
Variation in day length	Day length changes through the year, more at higher latitudes
Polar day and night	Inside the Polar Circles the Sun can stay up or down for 24 hours or more
Reversed seasons	The northern and southern hemispheres have opposite seasons

Without any tilt, the Sun would always be overhead at the equator, days and nights would be 12 hours everywhere, and there would be no seasons. If the tilt were larger, the seasons would be more extreme. Mars, with a similar tilt of about 25 degrees, also has seasons, while Uranus, tilted by about 98 degrees, has extreme ones.

Is the Tilt Constant?

The tilt is nearly constant over a human lifetime but it changes slowly over tens of thousands of years, and this is one of the Milankovitch cycles that influence the ice ages.

Obliquity (tilt): varies between about 22.1° and 24.5° over a cycle of about 41,000 years. The present value is about 23.4°, and is decreasing slowly.
Precession: the direction in which the axis points wobbles like a spinning top over about 26,000 years, so the pole star changes. Polaris is the pole star at present.
Eccentricity: the shape of the orbit changes from nearly circular to more elliptical over cycles of about 100,000 years.

Together these change the amount and distribution of sunlight and have helped to bring about the glacial and interglacial periods of the Quaternary.

How Does the Tilt Link to Heat and Pressure Belts?

Because the vertical rays of the Sun shift north and south during the year, the zone of greatest heating and the associated pressure and wind belts also shift. This seasonal swing explains the movement of the Inter-Tropical Convergence Zone and the Indian monsoon, and is explained in Insolation: Aphelion, Perihelion, Angle of Inclination of Sun’s Rays, Length of Day. The angle at which the rays strike the Earth also decides how intense the heating is, which is an essential concept for the topic on insolation and temperature.

Why Is This Topic Important?

Seasons Basis of climate studies
Latitudes Explains the Tropics and Polar Circles
Ice ages Milankovitch cycles

What Came Next?

The next topic explains how time is measured and fixed over the Earth, with local and standard time, the date line and calendars. Next topic: Local and Standard Time, International Date Line, Calendar.

Key People and Terms

Obliquity Tilt of the axis
Parallelism of the axis Axis keeps its direction in space
Precession Slow wobble of the axis
Eccentricity Shape of the orbit
Polaris Present pole star
Orbital plane Plane of the Earth’s orbit

Exam Corner

Points to Remember

The tilt is 23.5° from the normal and 66.5° from the orbital plane.
Tilt, not distance, causes seasons.
The axis keeps its direction: parallelism.
Obliquity cycle: about 41,000 years.
Seasons in the two hemispheres are opposite.

Do Not Confuse With

Tilt and distance as the cause of seasons Tilt is the cause
23.5° and 66.5° Angle with the normal versus angle with the orbital plane
Obliquity and precession Change in the angle versus wobble in the direction

Memory Hook

Tilt twenty-three and a half; the Sun swings, seasons sing.

Mains Angle

Questions ask for the effects of the tilt and the climate cycles. These points add depth.

Seasons Rays and day length
Zones Tropics and Polar Circles
Climate Milankovitch cycles

A Question You May Face

An original practice question, not a past paper question.

“What would happen to the seasons and the heat zones if the Earth’s axis were not tilted? Explain with reasons.”

How to Answer

State Present tilt and parallelism.
Without tilt No seasons, equal days.
Zones Tropics and Circles lose meaning.
Long term Milankovitch cycles.

GS Relevance

Prelims: tilt angle, parallelism, seasons, day length, Milankovitch cycles. GS Paper 1: motions of the Earth and their effects.

Frequently Asked Questions

What is the inclination of the Earth's axis?

The inclination of the axis is the tilt of the Earth's axis of rotation, which is about 23.5 degrees from the perpendicular to the orbital plane, or 66.5 degrees to the orbital plane. This tilt is the main cause of the seasons and of the changes in the length of day and night.

What is parallelism of the Earth's axis?

Parallelism of the axis means that, as the Earth revolves around the Sun, its axis stays tilted at the same angle and keeps pointing in the same direction in space, towards the pole star. This is why the northern and southern hemispheres lean towards the Sun at different times of the year.

What would happen if the Earth had no tilt?

If the axis were perpendicular to the orbit, the Sun would always be overhead at the equator, day and night would last about 12 hours everywhere, and there would be no seasons. The Tropics and Polar Circles would also not exist in the form we know.

What are Milankovitch cycles?

Milankovitch cycles are slow changes in the Earth's orbit and tilt: eccentricity of the orbit over about 100,000 years, obliquity of the axis over about 41,000 years, and precession over about 26,000 years. They change the sunlight reaching the Earth and are linked to the ice ages.

Why do the two hemispheres have opposite seasons?

Because of the tilt, when the northern hemisphere leans towards the Sun and gets direct rays and long days, the southern hemisphere leans away and gets slant rays and short days. Six months later the situation reverses, so the seasons of the two hemispheres are always opposite.

PYQ Practice — Statement Analysis

1 The Earth's axis is tilted at about 23.5 degrees to the normal to its orbital plane.

True

This is also 66.5 degrees to the plane.

2 Seasons are caused mainly by the change in distance between the Earth and the Sun.

False

They are caused by the tilt of the axis.

3 The obliquity cycle of the Earth lasts about 41,000 years.