Earth, Moon, and Sun System

Overview

This lecture explains the motions and interactions of the Sun, Earth, and Moon as a system, focusing on cycles, phases, eclipses, and tides.

Earth’s Motions

• Earth rotates on its axis every 24 hours, causing day and night.
• Earth revolves around the Sun in 365.25 days, leading to years and leap years.
• Earth’s axis is tilted 23.5°, causing seasonal changes.
• The ecliptic is the apparent path of the Sun and planets in the sky.
• The Earth’s tilt leads to varying sun elevation and daylight hours throughout the year.
• Earth’s axis wobbles slowly (precession), shifting the North Star over 26,000 years.

Seasons and Solar Angle

• Seasons are caused by Earth’s axial tilt, not its distance from the Sun.
• In summer, the hemisphere tilted toward the Sun receives higher solar angles and longer daylight.
• Solstices mark summer and winter starts; equinoxes mark spring and fall with equal day and night.
• The Tropic of Cancer and Capricorn mark where the Sun is directly overhead at solstices.

Earth’s Orbit and Eccentricity

• Earth’s orbit is elliptical, not perfectly circular.
• Perihelion: Earth closest to the Sun (early January).
• Aphelion: Earth farthest from the Sun (early July).
• Eccentricity measures how much an orbit deviates from a circle.

The Moon’s Movements and Phases

• The Moon rotates and revolves around Earth in about 27 days.
• The lunar cycle from new moon to new moon is ~29.5 days.
• The Moon’s orbit is elliptical: perigee (closest), apogee (farthest).
• The phases of the Moon are new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent.
• Waxing means increasing light; waning means decreasing light.
• The Moon always rises in the east and sets in the west.
• Libration is the slight wobble allowing us to see a bit more than half of the Moon’s surface.

Eclipses

• Solar eclipses: Moon blocks Sun from Earth, occur at new moon.
• Lunar eclipses: Earth blocks Sun from Moon, occur at full moon and Moon appears red.
• Total eclipses are rare due to the Moon’s orbit tilt; partial and annular eclipses occur based on distances.

Tides

• Tides are mainly caused by the Moon’s gravity, with some effect from the Sun.
• Two high and two low tides occur daily; tides are slightly later each day.
• Spring tides: higher tides when Sun, Moon, and Earth align.
• Neap tides: lower tides when Sun and Moon are at right angles to Earth.

Key Terms & Definitions

• Rotation — Spinning of a body on its axis.
• Revolution — Orbiting motion of a body around another.
• Ecliptic — Apparent path of Sun and planets in the sky.
• Axial Tilt — Angle between Earth’s axis and its orbital plane (23.5°).
• Precession — Slow wobble of Earth’s axis.
• Eccentricity — Degree of deviation of an orbit from circular.
• Perihelion/Aphelion — Closest/farthest point of Earth to/from the Sun.
• Perigee/Apogee — Closest/farthest point of the Moon to/from Earth.
• Waxing/Waning — Increasing/decreasing lunar illumination.
• Libration — Wobbling motion of the Moon.
• Solar/Lunar Eclipse — Blocking of sunlight by Moon/Earth.
• Spring/Neap Tide — Larger/smaller than normal tides based on Sun-Moon-Earth positioning.

Action Items / Next Steps

• Complete the assignment on Moon phases and their causes.
• Review diagrams of Earth–Sun–Moon interactions and eclipse geometry.
• Study key vocabulary for the next quiz.