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Rotation and Revolution
1. Introduction to Earth's Motions
- Like all planets in the Solar System, the earth revolves around the sun in an elliptical orbit and simultaneously rotates on its own axis.
- These two primary motions are known as rotation and revolution.
- They are responsible for phenomena such as day and night, changing seasons, and the behavior of winds and ocean currents.
- We do not feel these motions because the force of gravity keeps everything attached to the earth, and inertia (Newton's First Law of Motion) keeps us moving at a constant rate along with the earth.
2. Rotation of the Earth
- Definition: Rotation is the spinning of the earth on its axis.
- Direction: The earth rotates from west to east.
- Duration: One complete rotation takes 23 hours, 56 minutes, and 4.09 seconds (roughly 24 hours).
- The Axis: The imaginary line passing through the center of the earth, connecting the North Pole and South Pole. It is tilted at an angle of 23½° from the vertical, or 66½° to the plane of the earth's orbit.
- Speed: The rotational speed is maximum at the Equator (1670 km per hour) and gradually decreases towards the poles.
3. Effects of Rotation
- Day and Night: As the earth rotates, the part facing the sun experiences day, while the opposite side experiences night.
- Sunrise and Sunset: The west-to-east rotation causes the sun, moon, and stars to seemingly move across the sky from east to west.
- Coriolis Effect: An apparent force acting on moving objects over a rotating body. It causes winds and ocean currents to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
- Centrifugal Force: The outward force generated by rotation creates a slight bulge at the equator and a flattening at the poles.
- Time Differences: Rotation creates a time difference of 4 minutes for each degree of longitude across the globe.
- Temperature Moderation: The 24-hour cycle prevents extreme temperature variations between day and night, keeping the earth habitable.
4. Revolution of the Earth
- Definition: The movement of the earth around the sun in a fixed, elliptical orbit.
- Speed: The earth travels at an average speed of 29.8 kilometers per second (about 100,000 km per hour). Speed varies due to the gravitational pull, moving faster when closer to the sun.
- Duration: One complete revolution takes exactly 365 days, 5 hours, 48 minutes, and 45.51 seconds.
- Leap Year: For convenience, a standard year is 365 days. The remaining 6 hours are accumulated over four years to add one extra day (February 29th) creating a leap year with 366 days.
- Perihelion: The position when the earth is closest to the sun (about 147.3 million km), occurring around January 3rd.
- Aphelion: The position when the earth is farthest from the sun (about 152 million km), occurring around July 3rd.
5. Effects of Revolution & the Inclined Axis
- Varying Lengths of Day and Night: Because of the 23½° axial tilt, days and nights are unequal at different latitudes. If the axis were vertical, days and nights would be 12 hours everywhere, all year round.
- Seasonal Changes: The combination of the earth's revolution and its tilted axis causes seasons. The tilt determines whether a hemisphere is pointing towards or away from the sun.
- Creation of Heat Zones: The spherical shape of the earth and the angle of the sun's rays divide the earth into distinct temperature zones:
- Torrid Zone: Between 23½° N and S. Receives vertical sun rays; experiences a moderately warm climate year-round.
- Temperate Zone: Between 23½° and 66½° (N & S). Receives oblique sun rays; experiences a moderate climate.
- Frigid Zone: Between 66½° and 90° (N & S). Receives extremely oblique rays; experiences a very cold climate.
6. Solstices and Equinoxes
- Circle of Illumination: The line that divides the lighted part of the earth from the dark part.
- Solstice (Sun stands still): Occurs twice a year when the sun reaches its highest or lowest point in the sky at noon, creating the longest and shortest days.
- Summer Solstice (June 21): The North Pole is tilted towards the sun. Tropic of Cancer receives vertical rays. Longest day in the Northern Hemisphere.
- Winter Solstice (December 22): The South Pole is tilted towards the sun. Northern Hemisphere experiences winter and its shortest day.
- Equinox (Equal night): Occurs twice a year when the sun shines vertically over the Equator. Days and nights are exactly 12 hours long everywhere on earth.
- Spring / Vernal Equinox: March 21st.
- Autumnal Equinox: September 23rd.
7. Twilight and Dawn (Midnight Sun)
- Twilight / Dusk: The period of diffused light between sunset and complete darkness.
- Dawn: The period of diffused light between sunrise and full daylight.
- Duration Variations: Twilight is shortest at the Equator because the sun sets in a vertical path. It is longest near the poles due to the oblique path of the sun, sometimes lasting up to seven weeks.
- Midnight Sun: During the summer solstice, regions beyond the Arctic Circle (like Norway) experience continuous daylight for 24 hours or more, keeping the sun visible just above the horizon even at midnight.
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