Motions of the Earth: Rotation and Revolution and Their Effects

At a Glance

• Rotation West to east, 23 hours 56 minutes 4 seconds
• Revolution Around the Sun, 365 days 6 hours
• Orbit Elliptical
• Rotation causes Day and night, Coriolis effect, geoid
• Revolution causes Leap year and, with the tilt, seasons
• Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic covers the two basic motions of the Earth after the grid of latitudes and longitudes. Rotation gives the day and revolution gives the year, and between them they set up the calendar, the seasons and the pattern of winds and currents. Previous topic: Latitude and Longitude – Important Parallels and Meridians. Next topic: Inclination of the Earth’s Axis and Its Effects.

What Is the Rotation of the Earth?

In short: the Earth spins like a top on its axis, from west to east.

The spinning movement of the Earth is called rotation. The Earth rotates around its axis from west to east, which is why the Sun appears to rise in the east. The axis is the imaginary line that passes through the North Pole, the centre of the Earth and the South Pole. It takes approximately 24 hours, more exactly 23 hours 56 minutes and 4 seconds, to complete one rotation. The speed of rotation is greatest at the equator, about 1,670 km an hour, and falls to nearly zero at the poles. Days and nights occur because of the rotation, since only half of the Earth faces the Sun at any time. The circle that divides the day from the night on the globe is called the circle of illumination. The Earth’s axis is tilted: it makes an angle of 23.5 degrees with the normal, or 66.5 degrees with the orbital plane.

What Is the Shape of the Earth?

The Earth is not a perfect sphere. Its shape is called a geoid, although some sources call it an oblate spheroid. It is slightly flattened at the poles and bulges at the equator. The radius at the equator is larger than at the poles because the speed of rotation, and hence the centrifugal force, is greater at the equator than at the poles. The long-term effect of rotation has therefore pushed matter outward in the equatorial region.

Why Does Gravity Vary with Latitude?

The force of gravitation is not the same at different latitudes. It is greater near the poles and less at the equator, for two reasons. First, the poles are closer to the centre of the Earth because of the equatorial bulge, so the gravitational field there is stronger. Second, the speed of rotation is greater at the equator, so the centrifugal force is greater there. Centrifugal force and gravitation act in opposite directions, so gravity is slightly reduced at the equator compared with the poles. A person weighs slightly less at the equator than at the poles for this reason, although the difference is small.

What Are the Effects of Rotation?

• Day and night: the side facing the Sun has day and the other side has night, so every place on Earth has a daily cycle.
• Time: rotation gives the basis for local time and time zones, since the Earth turns 15 degrees in an hour.
• Deflection of winds and currents: because of the rotation, moving air and water are deflected to the right of their path in the northern hemisphere and to the left in the southern hemisphere. This is the Coriolis effect, and it shapes cyclones, planetary winds and ocean currents.
• Tides: the rotation of the Earth, with the pull of the Moon and the Sun, brings about the regular rise and fall of the sea level.
• Shape: the equatorial bulge and polar flattening arise from rotation.

The deflecting effect is covered further in Factors Affecting Wind: Pressure Gradient Force, Frictional Force, Coriolis Force and in Ocean Currents: Horizontal and Vertical Motion; Cold and Warm Currents.

Why Does Temperature Fall Towards the Poles?

Temperature falls at the surface of the Earth as one moves away from the equator towards the poles. This is because of the spherical or geoid shape of the Earth and the position of the Sun relative to it. The equator receives direct sunlight, while the Sun’s rays become slant or oblique as one moves poleward, so the energy received per unit area decreases. The details of this variation, with the heat zones, are given in Latitude and Longitude – Important Parallels and Meridians. As an analytical point, rotation alone does not explain the temperature pattern; the shape of the Earth and the angle of the Sun’s rays are the direct causes, while rotation shapes the day-night cycle and the deflection of moving fluids.

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 above.

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.

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.

Why Is This Topic Important?

• Day and night Basic cause
• Geoid and gravity Frequent Prelims concept
• Coriolis effect Foundation of winds and currents

What Came Next?

The next topic explains how the tilt of the axis turns the yearly revolution into seasons. Next topic: Inclination of the Earth’s Axis and Its Effects.

Key People and Terms

• Axis Line through the poles and the centre
• Circle of illumination Line dividing day from night
• Geoid Shape of the Earth, flattened at the poles
• Centrifugal force Outward force from rotation
• Coriolis effect Deflection of moving air and water
• Equatorial bulge Extra radius at the equator

Exam Corner

Mains Angle

Questions ask about effects of rotation. These points add depth.

• Time Day, night and time zones
• Shape Geoid and gravity differences
• Fluids Coriolis deflection