Earth, Moon, and Sun Relationships

Day and Night

• Earth is a sphere that rotates around an imaginary axis.
• One full rotation takes 24 hours.
• Hemisphere facing the Sun experiences daylight; the hemisphere facing away experiences night.
• Therefore, the day/night cycle is a direct result of Earth’s rotation on its axis.

Moon Phases

• The Sun continually illuminates half of the Moon’s surface.
• As the Moon orbits Earth, the portion of the lit half that we see changes, creating the phases.
• Key positions:
  • New Moon: Moon between Earth and Sun; lit side faces away—Moon appears dark.
  • Waxing: \text{crescent} \rightarrow \text{first\ quarter} \rightarrow \text{gibbous} —visible lit area increases.
  • Full Moon: Entire near side is illuminated.
  • Waning: \text{gibbous} \rightarrow \text{last\ quarter} \rightarrow \text{crescent} —visible lit area decreases.
• Cycle repeats, returning to New Moon.

Seasons

• Earth’s rotational axis is tilted by 23.5^\circ relative to its orbital plane.
• As Earth revolves around the Sun, this tilt causes varying sunlight angles and day lengths in each hemisphere:
  • Hemisphere tilted toward the Sun ➔ more direct sunlight & longer days ➔ summer.
  • Hemisphere tilted away from the Sun ➔ less direct sunlight & shorter days ➔ winter.
• Midway points, when both hemispheres receive nearly equal sunlight, produce spring (vernal) and autumn (fall).