3. Motions of the Earth

1. Introduction to Earth's Motions

The Earth experiences two primary types of motion simultaneously in space:

Rotation: The spinning of the Earth on its own axis, completing one full rotation in about 24 hours.
Revolution: The movement of the Earth around the Sun along its fixed orbit, completing one full revolution in about 365 days and 6 hours.

Concept

Inclination of the Earth's Axis: If you join the North and South Poles of the Earth with a straight line passing through its center, that is the axis. The axis of the Earth is not straight; it is tilted at an angle of 23½° from the perpendicular to the orbital plane. Thus, it makes an angle of 66½° with the plane of the Earth's orbit.

Important

The Leap Year Logic: The Earth takes 365 days and 6 hours to complete one revolution. For convenience, we consider a year as exactly 365 days. The remaining 6 hours are accumulated every year and add up to one full day (24 hours) over a span of four years. This extra day is added to the month of February, making it 29 days long instead of 28. This 366-day year is called a Leap Year.

2. Rotation of the Earth

The diurnal or daily motion of the spherical Earth spinning on its axis from West to East is known as rotation. This rotation creates the phenomenon of day and night.

The duration of the day is calculated as the interval of time between two successive passages of the mid-day Sun over a given position. The mean duration of a solar day is exactly 24 hours.
Speed of Rotation: At the Equator, the speed of rotation is maximum, about 1670 km/h (covering the 40,076 km circumference in 24 hours). The speed decreases as we go north or south, dropping to about 1120 km/h at 45° latitude, and becoming relatively nil at the Poles.
We do not feel this massive speed because the land, water, and atmosphere all move simultaneously with us at the same rate.

Concept

Circle of Illumination: Only half of the Earth is illuminated by the Sun at any given time. The imaginary line separating the lighted, day-time part of the Earth from the darker, night-time part is known as the Circle of Illumination.

Effects of the Earth's Rotation

The rotation of the Earth is responsible for several vital physical phenomena:

Movement of Celestial Bodies: The Sun, Moon, planets, and stars appear to move across the sky from East to West. This is an optical illusion caused by the Earth's actual West to East rotation.
Sunrise and Sunset: Rotation causes different places on Earth to experience sunrise and sunset at different times. The time when sunlight begins to appear before sunrise is Dawn, and the fading light after sunset is Dusk (or twilight).
Centrifugal Force: The rapid spinning generates a centrifugal force causing the Earth to bulge slightly at the Equator and flatten at the Poles, giving it an oblate spheroid shape.
Tides: The regular rising and falling of ocean waters (tides) occurs twice a day. While gravity from the Moon is the primary cause, the centrifugal force from rotation makes the tides sweep continuously around the Earth every 24 hours and 50 minutes.
Coriolis Effect: Earth's rotation causes freely moving bodies like winds and ocean currents to be deflected from their straight paths. They deflect to the right in the Northern Hemisphere (clockwise) and to the left in the Southern Hemisphere (anti-clockwise).
Occurrence of Day and Night: As the Earth spins, the half facing the Sun experiences day, and the half turned away experiences night.

3. Revolution of the Earth

The movement of the Earth around the Sun is called revolution. The Earth travels along a fixed, elliptical path known as its orbit.

Fact

Aphelion and Perihelion: Because the orbit is elliptical, the distance between the Earth and the Sun varies.
• Aphelion: Around July 4th, the Earth reaches its maximum distance from the Sun (about 152 million km).
• Perihelion: Around January 3rd, the Earth is at its closest distance to the Sun (about 147 million km).

Effect of the Tilted Axis on Day and Night

The axis of the Earth remains tilted in the same direction throughout its revolution. This inclination causes significant variations in the length of days and nights:

For six months, the Northern Hemisphere is inclined towards the Sun. A larger portion faces sunlight, causing longer days and shorter nights in the Northern Hemisphere. During this time, the Southern Hemisphere is tilted away, experiencing shorter days.
For the next six months, the situation reverses. The Southern Hemisphere tilts towards the Sun, experiencing longer days.
At the Poles: When tilted towards the Sun, the pole remains continuously in sunlight (6 months of day). When tilted away, it stays completely in darkness (6 months of night).
At the Equator: The length of days and nights remains completely equal (12 hours each) throughout the year because it is bisected evenly by the Circle of Illumination regardless of the tilt.

4. The Change of Seasons

The phenomenon of the changing seasons is caused by two combined factors:

The revolution of the Earth around the Sun.
The inclination of the Earth's axis at 66½° to its orbital plane.

Summer Solstice (21st June)

The North Pole is inclined heavily towards the Sun, and the South Pole is turned away.
The vertical rays of the Sun fall directly over the Tropic of Cancer (23½°N).
This position is known as the Summer Solstice. It brings Summer to the Northern Hemisphere (with its longest day of the year) and Winter to the Southern Hemisphere.

Winter Solstice (22nd December)

The South Pole is inclined towards the Sun, and the North Pole points away.
The vertical rays of the Sun shift south and fall directly over the Tropic of Capricorn (23½°S).
This position is known as the Winter Solstice. It brings Summer to the Southern Hemisphere (its longest day) and Winter to the Northern Hemisphere (its shortest day).

The Equinoxes (21st March and 23rd September)

"Equinox" means equal nights. On these two dates, neither pole is inclined towards or away from the Sun.
The vertical rays of the Sun fall exactly on the Equator (0°).
All places on Earth experience exactly equal days and equal nights (12 hours each).
21st March (Vernal/Spring Equinox): Marks the beginning of Spring in the Northern Hemisphere and Autumn in the Southern Hemisphere.
23rd September (Autumnal Equinox): Marks the beginning of Autumn in the Northern Hemisphere and Spring in the Southern Hemisphere.