Earth's Movements: Rotation, Revolution, and Seasonal Dynamics, and Seasonal Impacts, and Seasonal Variations

Introduction to Earth's Movements

• Perspective of Motion: The apparent movement of the Sun, Moon, Stars, and other celestial bodies often gives the false impression that they move while the Earth remains stationary. In reality, it is the Earth that is constantly in motion.

• Perception of Speed: Humans do not feel the rotation of the Earth because they are moving along with the planet at the same velocity or speed.

• Relativity of Motion: The daily observation of the Sun rising in the East, reaching its highest point or Zenith at mid-day, and setting in the West is an effect of the relativity of motion between the Sun and the Earth.

• Gravitational Stability: All heavenly bodies are in constant motion but are held together in space by gravitational forces. The Earth's gravitational force specifically holds the Moon and artificial satellites in their respective rotational paths or orbits.

• The Primary Movements: The Earth exhibits two primary movements: Rotation and Revolution.

Rotation of the Earth

• Definition: Rotation is defined as the spinning of the Earth on its imaginary Polar Axis.

• Direction of Rotation: The Earth rotates on its axis from West to East. This specific direction determines the apparent direction in which the Sun, Moon, and stars rise and set.

• Period of Rotation:

  • The time required for the Earth to turn through $360^{\circ}$ is precisely $23\,\text{hr}, 56\,\text{minutes}, 4.09\,\text{sec}$.

  • This specific time period is known as the Sidereal Day and is determined with respect to the stars.

  • For practical calendar purposes, a day is taken as $24\,\text{hours}$.

• The Circle of Illumination: This is an imaginary line that marks the clear demarcation between the phenomena of day and night on the Earth. It divides the Earth into a sunlit half and a shadowed half.

Effects of Rotation

• Occurrence of Day and Night: As the Earth rotates from west to east, the part of the planet facing the Sun experiences daylight, while the opposite side experiences night. For example, when it is morning in India, it is night in America.

• Coriolis Effect and Deflection: The rotation of the Earth creates a force known as the Coriolis force. According to Ferrell's Law, all moving bodies on the Earth's surface (such as ocean currents and winds) are deflected:

  • Towards the right in the Northern Hemisphere.

  • Towards the left in the Southern Hemisphere.

• Sunrise and Sunset: The West-to-East rotation is responsible for the apparent daily path of celestial bodies, causing the Sun to rise in the East and set in the West.

• Variation in Temperature: Areas receiving Sun's rays accumulate heat during the day, raising the temperature. At night, this heat is given off, causing the temperature to drop. This alternation prevents extreme temperature build-up.

• Centrifugal Force and Earth's Shape: Rotation creates a centrifugal force directed away from the axis, similar to a spinning top. This results in:

  • A bulge at the Equator.

  • Flattening at the poles.

  • This is caused by the differential speed of rotation across different latitudes.

• Occurrence of Tides: Sea water experiences an alternate rise and fall twice every $24\,\text{hours}$ due to the gravitational pull of the Sun and Moon. The specific interval between two tides is $12\,\text{hours}, 56\,\text{mins}$ due to the Moon's revolution around the Earth in the same direction as Earth's rotation.

The Coriolis Effect

• Discovery: First described in $1835$ by the French scientist Gustave Coriolis.

• Mechanism: The effect is a result of centrifugal force and the difference in rotating speed as one moves towards the poles.

• Latitudinal Relationship:

  • The Coriolis effect is absent ($0$) at the Equator.

  • As latitude increases, the speed of the Earth's rotation decreases, but the Coriolis effect increases.

  • The effect is strongest at the Poles.

• Hemispheric Deflection:

  • Northern Hemisphere: Deflection is to the right.

  • Southern Hemisphere: Deflection is to the left.

Revolution of the Earth

• Definition: The movement of the Earth along its elliptical orbit around the Sun is termed Revolution.

• Period of Revolution:

  • The Earth completes one revolution in approximately $365 \frac{1}{4}\,\text{days}$ (specifically $365.25\,\text{days}$).

  • Leap Year: To account for the extra $\frac{1}{4}$ day every year, one day is added to February every $4\text{th}$ year (or multiples of $4$). This corrects the calendar with respect to the tropical year.

• Distance from the Sun:

  • Mean Distance: Approximately $150\,\text{million km}$.

  • Perihelion: Occurs on $\text{January } 3\text{rd}$, when the Earth is at its closest point to the Sun, approximately $147\,\text{million km}$. (Greek: peri - around/near, helios - Sun).

  • Aphelion: Occurs on $\text{July } 4\text{th}$, when the Earth is at its farthest point from the Sun, approximately $152\,\text{million km}$. (Greek: ap - away from, helios - Sun).

• Speed of Revolution:

  • Mean Velocity: $1,07,000\,\text{km/hr}$ or $30\,\text{km/sec}$.

  • The velocity is not constant; it is greatest at Perihelion and least at Aphelion.

  • Direction: When viewed from space, the Earth travels counter-clockwise around the Sun.

Inclination of the Axis

• Definition of Axis: An imaginary line through the center of the Earth. The North end is the North Pole ($90^{\circ}\,\text{N}$) and the South end is the South Pole ($90^{\circ}\,\text{S}$).

• Angle of Inclination:

  • The axis is not perpendicular to the plane of the Earth's orbit (the plane of the ecliptic).

  • It makes an angle of $66\frac{1}{2}^{\circ}$ with the plane of the ecliptic.

  • It is tilted $23\frac{1}{2}^{\circ}$ from the line perpendicular to that plane.

• Parallelism of the Axis: The Earth's axis remains fixed at this angle and always points in the same direction (towards the Pole Star) throughout its revolution.

Factors Determining Seasons

• The alternation of seasons is the result of four main factors:

  1. The rotation of the Earth on its inclined axis.

  2. The revolution of the Earth around the Sun.

  3. The fixed position of the Earth's axis (pointing to the Pole).

  4. The angle (altitude) of the Sun's rays at noon.

• Impact of Inclination: Because the axis is tilted, each pole is inclined alternatively toward the Sun twice a year. If the axis were perpendicular ($90^{\circ}$) to the ecliptic, every place would have $12\,\text{hours}$ of day and $12\,\text{hours}$ of night, and there would be no seasons.

• Solar Energy Distribution: Direct rays are hotter than oblique rays. Oblique rays cover a larger surface area and lose more heat while passing through the atmosphere.

Solstices and Equinoxes

• Summer Solstice ($\text{June } 21$):

  • The North Pole is inclined toward the Sun.

  • The Sun is directly overhead at the Tropic of Cancer ($23\frac{1}{2}^{\circ}\,\text{N}$).

  • Northern Hemisphere experiences the longest day and shortest night.

  • The region beyond the Arctic Circle ($66\frac{1}{2}^{\circ}\,\text{N}$) experiences literal "Midnight Sun" for $24\,\text{hours}$.

• Winter Solstice ($\text{December } 22$):

  • The South Pole is inclined toward the Sun.

  • The Sun is directly overhead at the Tropic of Capricorn ($23\frac{1}{2}^{\circ}\,\text{S}$).

  • Northern Hemisphere experiences its shortest day and longest night (Winter).

  • The area beyond the Arctic Circle experiences $24\,\text{hours}$ of darkness.

• Equinoxes:

  • Vernal/Spring Equinox ($\text{March } 21$) and Autumnal Equinox ($\text{September } 23$).

  • The Sun is directly overhead at the Equator.

  • The Circle of Illumination coincides with the meridians, passing through both poles.

  • Every place on Earth experiences exactly $12\,\text{hours}$ of day and $12\,\text{hours}$ of night. (Latin: aequus - equal, nox - night).

Varying Length of Day and Night

• Cause: While day and night are caused by rotation, their respective durations are caused by the inclination of the axis and the Earth's position in its orbit.

• Northern Hemisphere Summer ($\text{June } 21$): Day length increases with latitude toward the North Pole. Beyond the Arctic Circle, day length is $24\,\text{hours}$.

• Southern Hemisphere Summer ($\text{December } 22$): Day length increases with latitude toward the South Pole. The South Pole has continuous daylight for six months.

• Equatorial Stability: Near the Equator, the apparent movement of the Sun varies very little. Consequently, days and nights remain nearly equal ($12\,\text{hours}$ each) throughout the year.

Assess Yourself: Questions & Discussion

• Q: State two effects of west to east rotation of the earth.

  • A: 1. Occurrence of day and night. 2. Deflection of winds and ocean currents due to Coriolis force.

• Q: What causes the formation of day and night? Explain with the help of a neat diagram.

  • A: Day and night are caused by the Earth's rotation on its axis. As the Earth rotates, the half facing the Sun receives light (day), while the half facing away is in shadow (night). The Circle of Illumination is the boundary between the two.

• Q: Why is there a difference in the period of rotation ($23\,\text{hr}, 56\,\text{min}$ vs $24\,\text{hours}$)?

  • A: The $23\,\text{hr}, 56\,\text{min}$ duration is the Sidereal Day (relative to stars), while the $24\,\text{hour}$ day is the Solar Day (relative to the Sun).

• Q: What would happen if earth's axis was vertical instead of inclined?

  • A: All places on Earth would have exactly $12\,\text{hours}$ of day and $12\,\text{hours}$ of night daily, and there would be no seasonal changes.

• Q: On which two days does the circle of Illumination coincide with meridians and why?

  • A: On $\text{March } 21\text{st}$ and $\text{September } 23\text{rd}$ (Equinoxes), because the Sun is directly overhead at the Equator.