Earth's Motion Around the Sun

Earth's Motion

• Earth spins (rotation) and moves around the Sun (revolution).

• These motions cause day/night cycles and seasonal changes.

Earth's Rotation

• Rotation: Spinning motion on an imaginary line (rotation axis).

• Earth rotates counterclockwise (west to east).

• One rotation takes about 24 hours, completing the day and night cycle.

• Daytime: Half of Earth facing the Sun.

• Nighttime: Half of Earth facing away from the Sun.

Earth's Revolution

• Revolution: Motion of one object around another.

• Earth revolves around the Sun every 365.24 days.

• Orbit: The path an object follows during revolution; Earth's orbit is nearly circular.

• Gravity: The Sun's gravity keeps Earth in orbit.

  • Stronger with greater mass and closer distance.

• Without the Sun's gravity, Earth would move in a straight line.

Earth's Tilted Axis

• Earth's rotation axis is tilted and remains constant.

• One half of Earth's orbit: the north end of the axis tilts toward the Sun.

• Other half of Earth's orbit: the north end of the axis tilts away from the Sun.

Apparent Motion

• The Sun appears to move from east to west due to Earth's west-to-east rotation.

• The Moon and stars also appear to move similarly.

Temperature and Earth's Curved Surface

• Air temperature is hotter near the equator and colder near the poles.

• Sunlight carries energy, and the amount received depends on the angle of the surface.

• Due to Earth's curvature, the Sun's energy is more spread out near the poles and less spread out near the equator.

Earth's Seasons

• Seasons are not due to Earth's distance from the Sun.

• They are caused by the tilt of Earth's rotation axis and Earth's revolution around the Sun.

• Northern Hemisphere:

  • Tilted away from the Sun: Fall and Winter (less energy).

  • Tilted toward the Sun: Spring and Summer (more energy).

Earth's Seasonal Cycle

• Earth's rotation axis always points in the same direction in space.

• Solstice: Earth’s rotation axis is most toward or away from the Sun.

• Equinox: Earth’s rotation axis is leaning along Earth’s orbit, neither toward nor away from the Sun.

Solstices and Equinoxes

• December Solstice (December 21 or 22):

  • North end of Earth’s rotation axis is away from the Sun.

  • Northern Hemisphere: shortest days, longest nights, winter begins.

  • Southern Hemisphere: longest days, shortest nights, summer begins.

• September Equinox (September 22 or 23):

  • 12 hours of daylight and darkness everywhere.

  • North end of Earth’s rotation axis leans along Earth’s orbit.

  • Northern Hemisphere: autumn begins.

  • Southern Hemisphere: spring begins.

• March Equinox (March 20 or 21):

  • 12 hours of daylight and darkness everywhere.

  • North end of Earth’s rotation axis leans along Earth’s orbit.

  • Northern Hemisphere: spring begins.

  • Southern Hemisphere: autumn begins.

• June Solstice (June 20 or 21):

  • North end of Earth’s rotation axis is toward the Sun.

  • Northern Hemisphere: longest days, shortest nights, summer begins.

  • Southern Hemisphere: shortest days, longest nights, winter begins.

Changes During the Year

• March Equinox to June Solstice: North end of the rotation axis points more toward the Sun; Northern Hemisphere receives more solar energy (spring).

• June Solstice to September Equinox: North end of the rotation axis points less toward the Sun; Northern Hemisphere receives less solar energy (summer).

• September Equinox to December Solstice: North end of the rotation axis points away from the Sun; Northern Hemisphere receives less solar energy (fall).

• December Solstice to March Equinox: North end of the rotation axis points less away from the Sun; Northern Hemisphere receives more solar energy (winter).

Sun's Apparent Path

• The Sun’s apparent path across the sky changes with the seasons.

• Highest in the Northern Hemisphere on the June solstice and lowest on the December solstice.