Rotation and Revolution Chapter Notes | Footprints Class 6: Book Solutions, Notes & Worksheets PDF Download

At any given moment, thanks to the Earth's rotation, we're all traveling at an astonishing speed of approximately 1,674 kilometers per hour.

The Earth's motion involves two distinct types:

1. Rotation: This is the Earth's spinning movement around its imaginary axis. It's what causes day and night as different parts of the Earth face towards or away from the Sun.
2. Revolution: This is the Earth's orbital movement around the Sun. It takes one year for the Earth to complete one full orbit around the Sun.

Rotation

The Earth spins or rotates from west to east, in a counter-clockwise direction, on its axis, which is an imaginary straight line that runs through the center of the Earth.

• The axis of the Earth is inclined: at an angle of 23°20' away from a line drawn perpendicular to the plane of its orbit around the Sun, or 66°20' to the orbital plane itself.
• It is always tilted in the same direction.

Effects of Rotation

The Earth's rotation on its axis, completing a single rotation in approximately 24 hours or one day, plays a crucial role in determining day and night. This rotation, combined with the tilt of the Earth's axis, affects the distribution of sunlight across the planet.

• The Earth's rotation from west to east causes the Sun to appear to rise in the east and set in the west. 
• At sunrise, the part of the Earth facing the Sun experiences daylight, while at sunset, it turns away from the Sun, entering its own shadow and causing night to fall.
• The boundary between areas experiencing daylight and those in darkness is known as the circle of illumination. Notably, this circle does not align perfectly with the Earth's axis.
• Understanding these effects helps us comprehend the cyclical pattern of day and night on Earth, driven by its rotation relative to the Sun.

Revolution

The Earth not only rotates on its axis but also revolves around the Sun in its orbit. This revolution occurs at an impressive speed of approximately 107,275.57 kilometers per hour.

• As the Earth travels along its elliptical orbit around the Sun, it experiences varying distances from the Sun. 
• At certain times of the year, the Earth is closest to the Sun, known as perihelion, while at other times, it is farthest from the Sun, known as aphelion.
• It takes the Earth approximately 365 days and 6 hours to complete one full revolution around the Sun. These 365 days and 6 hours constitute one year, marking the completion of a full cycle of seasons and celestial events.

Effects of Revolution

The combination of the Earth's tilted axis and its revolution around the Sun results in unequal lengths of day and night, as well as the occurrence of seasons.

• Unequal Lengths of Day and Night: Observing, it's evident that the Earth maintains the same tilt angle throughout its orbit around the Sun. 
• This tilt causes different parts of the Earth to receive varying amounts of sunlight at different times of the year. Only at the Equator are days and nights always of equal duration throughout the year.
• The positions of the Earth in its orbit correspond to specific times of the year, such as the solstices and equinoxes. 
• These astronomical events mark significant changes in the duration of daylight and darkness experienced at different latitudes across the globe.

Seasons

The occurrence and change of seasons are primarily influenced by two factors:

1. Revolution of the Earth around the Sun: As the Earth orbits the Sun, different parts of the planet receive varying amounts of sunlight, leading to unequal lengths of day and night. This variation in sunlight exposure contributes to the transition between seasons.

2. Tilt of the Earth's axis: The Earth's axis is tilted relative to its orbit around the Sun. This tilt remains consistent throughout the Earth's revolution. As a result, different parts of the Earth receive direct sunlight at different angles throughout the year, leading to the changing seasons.

The Earth experiences four distinct seasons—spring, summer, autumn (fall), and winter—over the course of a year. These seasons occur at opposite times in the two hemispheres due to the Earth's axial tilt:

• When the Northern Hemisphere is tilted towards the Sun, it experiences summer, while the Southern Hemisphere is tilted away, experiencing winter.
• Conversely, when the Southern Hemisphere is tilted towards the Sun, it experiences summer, while the Northern Hemisphere is tilted away, experiencing winter.

This opposite tilt pattern between the two hemispheres results in their experiencing opposite seasons at any given time of the year.

Change in Seasons

Changes in seasons can be explained by two phenomena called solstice (meaning 'Sun standing still') and equinox (meaning 'equal nights').

Solstice

Summer Solstice: On 21 June, the North Pole is tilted towards the Sun and the Sun's rays fall directly on the Tropic of Cancer.

• This position of the Earth is known as the summer solstice, as it is summer in the Northern Hemisphere.
• It is the longest day in the Northern Hemisphere.
• During the summer time, the days are longer and the nights are shorter in the Northern Hemisphere.
• The region between the Arctic Circle and the North Pole receives sunlight 24 hours a day for a few months, as the Sun never sets.
• During this time, it is winter in the Southern Hemisphere, where nights are longer than the days.
• The region between the Antarctic Circle and the South Pole is always in darkness for a few months and does not see the Sun.
  

Winter Solstice: On 22 December, the Sun's rays fall vertically on the Tropic of Capricorn. The South Pole is tilted towards the Sun, and the Tropic of Capricorn receives the direct rays of the Sun.

• This position of the Earth that falls on 22 December is known as the Winter solstice, as it is winter in the Northern Hemisphere.
• Nights are longer, and days are shorter now in the Northern Hemisphere.
• Between the Arctic Circle and the North Pole, it is darkness for 24 hours for a few months, as the Sun is not seen.
• During this time, the Southern Hemisphere experiences summer as the vertical rays of the Sun are on the Tropic of Capricorn.
• Days are longer than the nights in the Southern Hemisphere at this time, and 22 December is the longest day there.
• Places between the Antarctic Circle and the South Pole experience 24 hours of daylight for a few months.

Equinox

Equinoxes, derived from Latin meaning "equal nights," mark two significant points in the Earth's orbit around the Sun:

Autumnal Equinox

As the Earth transitions from its position in June, the duration of sunlight gradually decreases in the Northern Hemisphere until September 23rd. 

• On this day, the Sun's rays are directly overhead the Equator, marking the autumnal equinox. 
• During this time, day and night are of equal length (12 hours each) across the globe. In the Northern Hemisphere, autumn begins, characterized by moderate temperatures and cooler weather. 
• September and October are typically considered autumn months in the Northern Hemisphere, while in the Southern Hemisphere, it is spring.

Vernal or Spring Equinox

Following December, winter begins to recede in the Northern Hemisphere as the Earth moves northward from the Southern Hemisphere to its position in March. 

• On March 21st, the Sun shines directly over the Equator, marking the vernal or spring equinox. 
• Once again, day and night are of equal length globally. In the Northern Hemisphere, this signals the onset of spring, with temperatures gradually warming. 
• March and April are typically considered spring months in the Northern Hemisphere, while in the Southern Hemisphere, it is autumn.