• Earth stands out as the most extraordinary planet in our solar system, primarily because it sustains life. Its unique features, including the distribution of land and water and the gradual shift of seasons, contribute to its exceptional status. The fundamental aspects of life on Earth are intricately tied to its rotation and revolution.

Introduction

• The Earth's axis, an imaginary line, forms an angle of 66½° with its orbital plane—the plane created by its orbit around the Sun.
• Illumination on Earth is a result of sunlight from the Sun. Due to the spherical shape of the Earth, only one half receives sunlight at any given time. The side facing the Sun experiences day, while the opposite side undergoes night.
• Earth's spherical shape ensures that half of its surface is exposed to sunlight while the other half remains in darkness. Consequently, one part experiences day, and the other experiences night. The demarcation between day and night on Earth is known as the Circle of Illumination.

Types of Earth's Motion

• Earth, the lone habitable planet in our solar system, exhibits two distinct motions: rotation (around its axis) and revolution (around the Sun). Let's delve into these motions:

Rotation

• Rotation is Earth's motion along its axis, an imaginary line extending from the North Pole to the South Pole. The Earth rotates from West to East, causing the Sun, Moon, and stars to appear to rise in the East and set in the West.
• The Earth completes one full rotation around its axis in 23 hours, 56 minutes, and 4 seconds, constituting the Earth-day or Sidereal day. This duration represents the time required for Earth to rotate 360° on its axis.
• The rotational speed varies across latitudes, with the velocity decreasing from the equator towards the poles. At the equator, the rotational speed is 1700 km/h. Earth's rotation induces a deflection in ocean and air currents due to the Coriolis force, causing winds to deflect to the right in the Northern hemisphere and to the left in the Southern hemisphere. This rotational effect also leads to the formation of gyres, large rotating pools of water in the oceans.
• Earth's rotation has several consequential effects:
  • It creates a diurnal cycle of light and darkness, resulting in temperature and humidity changes.
  • It causes the day-night cycle.
  • It influences tides, leading to the twice-daily rise and fall of sea levels.
  • Revolution, on the other hand, is the motion of the Earth along its elliptical orbit around the Sun, taking 365.4 days (one year) to complete. To account for the surplus six hours each year, an extra day is added to February every four years, creating a leap year with 366 days.

Revolution

• During its revolution, Earth travels an average distance of approximately 150 million km around the Sun. 
• The speed of revolution is about 29.6 km per second or 107,000 km/hr.
• This revolution is responsible for the occurrence of seasons on Earth. Earth's orbital path is an ellipse, causing variations in its distance from the Sun. 
• Perihelion, the closest point to the Sun (147 million km), occurs around January 3rd, while aphelion, the farthest point (152 million km), occurs around July 4th.
• The consequences of Earth's revolution include:
  • Changing seasons across hemispheres due to the tilt of the Earth's axis.
  • Influence on the development of water in seas and the direction of tides.
  • Impact on wind patterns, redirecting winds and ocean currents in the Northern and Southern hemispheres.

Changing seasons 

• The changing seasons result from the tilt of Earth's axis during its revolution around the Sun. Different inclinations of the Sun's rays at various times in the same location lead to variations in heat distribution, causing summer or winter seasons on Earth. The main stages experienced due to Earth's revolution include:

Summer Solstice (June 21st):

• The Northern hemisphere tilts towards the Sun.
• Sun's rays directly fall on the Tropic of Cancer (23.5° latitude North), resulting in areas receiving more heat.
• Areas near the poles receive less heat as the Sun's rays are slanting.
• The North pole is inclined towards the Sun, and places beyond the Arctic circle experience continuous daylight for about six months.
• Since a large portion of the Northern hemisphere is getting light from the Sun, it is summer in the regions North of the equator. The longest day and the shortest night at these places occur on 21st June.
• At this time in the Southern hemisphere, all these conditions are reversed. It is winter there. The nights are longer than the days. This position of the Earth on 21st June is called the Summer Solstice in the Northern hemisphere

Winter Solstice

• On December 22nd, the Tropic of Capricorn receives direct rays from the Sun as the South pole tilts towards it. The Sun's rays fall vertically at the Tropic of Capricorn (23.5° South), illuminating a larger portion of the Southern hemisphere.
• Consequently, it is summer in the Southern hemisphere with longer days and shorter nights. In the Northern hemisphere, the reverse occurs, marking the winter season with shorter days than nights. This Earth position is known as the Winter Solstice.

Heat Distribution on Earth

• When vertical rays fall on a specific part of the Earth, the absorption of heat is greater. Conversely, when oblique rays of the Sun hit a region, the absorption of heat is reduced. It is a well-known fact that there is more heat in the afternoon and less heat in the morning and evening. This is because the Sun's rays directly hit the surface in the afternoon, while they are oblique in the morning and evening.

Equinox

• On March 21st and September 23rd, direct rays of the Sun fall on the equator. During this alignment, neither pole is tilted towards the Sun, resulting in equal day and night duration across the entire Earth. This phenomenon is known as an equinox.
• On September 23rd, it is autumn in the Northern hemisphere and spring in the Southern hemisphere, referred to as the autumn equinox. Conversely, on March 21st, it is spring in the Northern hemisphere and autumn in the Southern hemisphere, known as the vernal or spring equinox.

Varying Length of Day and Night

• The Earth's axis is inclined to the plane of the ecliptic at an angle of 66.5°. This inclination contributes to different seasons and varying lengths of day and night throughout the year.

Eclipses

• An eclipse occurs when a celestial body obstructs the Sun's light. There are two types of eclipses: solar and lunar.

Solar Eclipse

•  A solar eclipse takes place near the New Moon position when the Moon is between the Sun and Earth, blocking a portion or the entirety of the Sun as seen from Earth.
• Conjunction, when the Sun and Moon align on one side of the Earth, precedes a solar eclipse.
• Unlike a solar eclipse, which is visible only from a specific area of the world, a lunar eclipse can be observed from anywhere on the Earth's night side.
• During a total eclipse, the Moon's shadow is short enough to cover the entire Sun, resulting in the outer region of the Sun glowing and appearing bright, resembling a ring—referred to as a diamond ring.
• There are four types of solar eclipses: total, annular, partial, and hybrid.

Solar Eclipse

• Total Solar Eclipse
• Annular Solar Eclipse
• Partial Solar Eclipse
• Hybrid Solar Eclipse

1. Total Solar Eclipse

•  Total solar eclipse occurs, when the New Moon comes between the Sun and Earth and casts the darkest part of its shadow, the Umbra on Earth. A full solar eclipse known as totality is almost as dark as night.
• During a total eclipse of the Sun, the Moon covers the entire disk of the Sun. It is called total eclipse because at the maximum point of the eclipse (midpoint of time of totality), the sky goes dark and temperatures can fall.

• The total eclipse has a brief duration, with the longest recorded total eclipse lasting only up to 7 minutes.
• The deep, dark region of the Sun or Moon covered by the vertical shadow of a celestial body, such as Earth or Moon, is known as Umbra, while the faint shadow region is referred to as Penumbra.

2. Annular Solar Eclipse

• An annular solar eclipse occurs when the Moon moves between the Sun and Earth but is at or near its farthest point from Earth.
• Due to the Moon's increased distance, it appears smaller than the Sun and fails to completely cover it.
• The annular solar eclipse typically lasts for around 12 minutes or more, sometimes resembling a total eclipse.

3. Partial Solar Eclipse

• A partial solar eclipse takes place when the Moon passes between the Sun and Earth, but the three celestial bodies are not perfectly aligned.
• Only a portion of the Sun is obscured, creating a crescent shape. Locations farther from the poles under the Penumbra zone cannot witness the complete eclipse.

4. Hybrid Solar Eclipse

• Due to the curved surface of the Earth, an eclipse may transition between annular and total phases as the Moon's shadow traverses the globe.

Lunar Eclipse

• A lunar eclipse occurs near the Full Moon position, with the Earth situated between the Sun and the Moon, casting a shadow on the lunar surface.
• The Moon's orbital plane is inclined 5° to the Earth's orbital plane, preventing eclipses from happening at every Full Moon or New Moon.
• Syzygy is the alignment position when Earth aligns in a straight line between the Sun and the Moon, resulting in a lunar eclipse.
• Lunar eclipses last for several hours, whereas total solar eclipses have a much shorter duration due to the Moon's smaller shadow. Lunar eclipses are more frequent than solar eclipses.
• There are three types of lunar eclipses: total lunar eclipse, penumbral lunar eclipse, and partial lunar eclipse.

1. Total Lunar Eclipse

• A total lunar eclipse occurs when the Earth is positioned between the Sun and the Moon, casting its shadow on the lunar surface.
• While the Moon doesn't vanish completely, the darkness may create confusion about its visibility.
• Additionally, sunlight refracted from the Earth's surface gives the Moon a faint glow.
• Total lunar eclipses occur approximately once every year and a half, according to NASA (National Aeronautics and Space Administration).

2. Partial Lunar Eclipse

• An imperfect alignment of Sun, Earth and Moon results in the Moon passing through only part of Earth's umbra.
• The shadow grows and then recedes without ever entirely covering the Moo

3. Penumbral Eclipse

• The Moon travels through Earth's penumbra or the faint outer part of its shadow.
• The Moon dims so slightly that it can be difficult to notice.
• A maximum of 7 eclipses can occur in a year including both solar and lunar eclipses. They do not take place on every Full Moon and New Moon
• This is because the Moon's orbit is tilted at about 5 degrees to Earth's orbit 

Super Moon, Blue Moon and Blood Moon

• Super Moon When the Moon is at the minimum distance to the Earth it is referred as Super Moon. It is also referred to as a perigee or Full Moon. The Moon appears 14% larger and 30% brighter. The last SuperMoon lunar eclipse occurred in 1982 and next won't take place until 2033.
• Blue Moon  In the case of two Full Moons in a calendar month, the second Moon is referred to as a Blue Moon. Blue Moon year refers to the occurrence of a Blue Moon in two or more months in a particular year. 2018 was a Blue Moon year.
• Blood Moon  Four lunar eclipses that occur in a row are called Blood Moon. This term comes from the reddish glow of a Full Moon, which takes place when the Moon passes between Earth and Sun, blocking the Sun's light.

Prelims Facts

• The balance between the upstanding and low lying parts of the Earth crust is known as
  • -Isostasy (BPSC (Pre) 20221
• The relation of the Earth about its axis affects the direction of the wind and this force is called
  • Coriolis Force (BPSC (Pre) 2022)
• In the Northern hemisphere, the longest day of the year normally occurs in the
  • - Second half of the month of June [IAS (Pre) 2021)
• The diamond ring God's eye and Baily's beads are the parts of
  • -Solar eclipse (BPSC (Pre) 2018]
• The total length of Earth's Equator is about
  • 40,000 km (UKPSC (Pте) 2010]
• When the Earth is at the minimum distance from the Sun, then this position is called
  • - Perihelion [LAS (Pre) 2010]
• The circle which divides the day and night on Earth is known as
  • - Circle of illumination [UPPSC (Pre) 2006
• When the Sun, Earth and the Moon are aligned in a straight line, this position is called
  • -Syzygy (UPPSC (Pre) 2008]
• The Earth rotates on its axis at an inclination of
  • 23.5° (UPPSC (Pre) 2016]
• The innermost and darkest part of a shadow, when the light source is completely blocked by the occluding body is called
  • Umbra [IAS (Pre) 2020)
• What causes wind to deflect towards the left in the Southern hemisphere ?
  • Rotation of the Earth [IAS (Pre) 2010)
• The Earth rotates on its axis at an inclination of 
  • 23.5° (UPPSC (Pre) 2016]
• In completing one revolution of the Sun, Earth takes approximately
  • -365.5 days [BPSC (Pre) 1996]
• Variations in the length of day time and night time from season to season are due to
  • revolution of the Earth on a tilted axis [IAS (Pre) 2013
• The Earth rotates on its axis in which direction?
  • - West to East (CGPSC (Pre) 2016

Self Check

1. The time taken by the Earth to revolve around the Sun

(a) 364 days

(b) 365 days

(c) 364 days

(d) 30 days

2. On what date does the summer solstice fall during the year when the path of the Sun in the sky is the farthest North in the Northern hemisphere?

(a) 20th or 21st August

(b) 20th or 21st July

(c) 20th or 21st June

(d) 2nd or 3rd June

3. What causes the change of seasons?

(a) Earth's rotation

(b) Earth's revolution

(c) Earth's revolution and inclination of its axis

(d) Earth's rotation and inclination of its axis

4. The movement of the Earth around the Sun is known as

(a) rotation

(b) revolution 

(c) inclination

(d) eccentricity

5. Consider the following statements and select the correct answer by using the codes given below.

UPPSC (Pre) 2000

1. The Northern end of the Earth's axis is called 'North pole".
2. 45" latitude is half of the length of the equator.
3. Earth's axis is parallel.
4. The Earth's speed of revolution is faster in aphelion position.

Codes

(a) 1 and 2

(b) 2 and 3

(c) 3 and 4

(d) 1 and 3

6. To a perpendicular to the plane of the ecliptic, the Earth's axis of rotation makes an angle of 23 1⁄2°, had this angle been 0°, which one among the following would result?

(a) There would have been no seasons.

(b) The length of day and night would have been the same throughout the year.

(c) The length of day and night would have been the same all over the Earth.

(d) All of the above

7. The equatorial region has no other season except summer. What could be the reason?

1. The length of day and night is more or less equal over the year.
2. The Earth's rotational velocity is maximum at the equator
3. The Coriolis force is zero at the equator.

Codes

(a) Only 1

(b) 1 and 2

(c) 2 and 3

(d) All of the above

8. The summer and winter seasons in a year are caused by

(a) aphelion (farthest) and perihelion (nearest) positions of the Earth from the Sun during the

annual revolution.

(b) rotation of the Earth on its axis.

(c) variation in solar insolation.

(d) revolution of the Earth on its inclined axis.

9. Why do we have a leap year after every 4 years? 

(a) The Earth gets shifted out of orbit every 4 years.

(b) The revolution slows down a little once every 4 years.

(c) The length of a year is not an integer number of days

(d) It is a convention.

10. When does a lunar eclipse happen? UPPSC (Pre) 2010 

(a) When the Moon comes between the Earth and the Sun.

(b) When the Earth comes between the Sun and the Moon.

(c) When the Sun comes between the Earth and the Moon.

(d) None of the above

11. When the Moon size is half of the Full Moon, the angle between the Sun, the Earth and the Moon is

UPPSC (Pre) 2005

(a) 45°

(b) 90°

(c) 180°

(d) 270°

Know Right Answer

1. (b)
2. (c)
3. (c)
4. (b)
5. (d)
6. (a)
7. (a)
8. (d)
9. (c)
10. (b)
11. (b)