Astronomy Chapter 1: The Motion of the Earth

Equinox and Solstice

• "Equinox" derives from Latin "Aequinoctium", meaning "equal nights" when the Sun is overhead at the Equator.
• Spring Equinox (Vernal Equinox) occurs on March 21 in the Northern Hemisphere; it is the Autumn Equinox in the Southern Hemisphere.
• September 23 marks the Autumnal Equinox in the Northern Hemisphere, while the Southern Hemisphere celebrates the Spring Equinox.

Important Dates

• March 21: Vernal Equinox
• June 22: Summer Solstice (Northern Hemisphere)
• September 23: Autumnal Equinox
• December 22: Winter Solstice (Northern Hemisphere)

Earth’s Characteristics

• Circumference of the Earth is approximately 40,000 km.
• Earth’s speed of revolution around the Sun is about 30 km per second.
• At Aphelion (around July 3), Earth is 152 million kilometers from the Sun.
• At Perihelion (around January 3), Earth is 147 million kilometers from the Sun.

Seasons and Their Oppositions

• The revolution and axial tilt of the Earth result in varying lengths of day and night, leading to seasonal changes.
• Sun's vertical rays determine solstices:
  • Northern Hemisphere experiences Summer Solstice on June 21 and Winter Solstice on December 22.
  • Southern Hemisphere experiences Summer Solstice on December 22 and Winter Solstice on June 21.

Earth’s Axial Motion

• Earth rotates on its axis and revolves around the Sun, with rotation taking approximately 24 hours to complete.
• The rotation speed is fastest at the Equator (about 1670 km/h) and decreases towards the poles (about 1666 km/h).
• Differential rotation speed contributes to the Earth's shape, causing it to bulge at the Equator and flatten at the poles.

Characteristics of Revolution

• Earth revolves around the Sun in an anti-clockwise direction, completing one revolution in 365 ¼ days.
• The Earth’s orbit is elliptical, causing varying distances from the Sun throughout the year, influencing seasonal temperature changes.