Overview
This lecture covers the interactions and motions of the Sun, Earth, and Moon as a system, focusing on rotation, revolution, tilt, lunar phases, eclipses, and tides.
Major Motions of Earth
- Earth rotates on its axis every 24 hours, causing day and night.
- Earth revolves around the Sun every 365.25 days, defining a year and causing leap years.
- Earth's rotation is counterclockwise (west to east) as seen from above the North Pole.
- The 23.5° tilt of Earth's axis causes the seasons.
- The ecliptic is the apparent path of the Sun and planets in the sky.
- The Earth’s axis points toward Polaris, but this changes over 26,000 years (precession).
Effects of Tilt & Seasons
- If Earth were not tilted, there would be no seasons.
- Summer occurs when a hemisphere is tilted toward the Sun; winter when tilted away.
- The angle of sunlight affects daylight hours and temperature.
- Solstices (start of summer/winter) and equinoxes (start of spring/fall) mark seasonal changes.
- Tropic of Cancer (23.5°N) and Tropic of Capricorn (23.5°S) receive direct sunlight at solstices.
Earth's Orbit Characteristics
- Earth’s orbit is elliptical, not perfectly circular.
- Perihelion: Earth is closest to the Sun (early January).
- Aphelion: Earth is farthest from the Sun (early July).
- Eccentricity measures how much an orbit deviates from a circle.
Motions and Phases of the Moon
- Moon rotates and revolves around Earth in ~27 days; lunar cycle (phases) is 29.5 days.
- Moon phases: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent.
- Waxing means increasing light (right side lit); waning means decreasing (left side lit).
- The same side of the Moon always faces Earth.
- Perigee: Moon closest to Earth; Apogee: Moon farthest.
Eclipses
- Solar eclipse: Moon passes between Earth and Sun, occurs at new moon.
- Lunar eclipse: Earth passes between Sun and Moon, occurs at full moon.
- Eclipses are rare because the Moon's orbit is tilted relative to Earth's orbit.
Tides
- Tides are mainly caused by the Moon's gravity, with Sun's gravity also contributing.
- Spring tides: Sun, Moon, Earth aligned, causing higher tides.
- Neap tides: Sun, Moon, Earth at right angles, causing lower tides.
- There are two high tides and two low tides each day.
Key Terms & Definitions
- Rotation — spinning of a body on its axis.
- Revolution — movement of a body around another (e.g., Earth around Sun).
- Ecliptic — path the Sun appears to follow across the sky.
- Precession — slow change in direction of Earth's axis.
- Perihelion — point when Earth is closest to the Sun.
- Aphelion — point when Earth is farthest from the Sun.
- Eccentricity — measure of orbit's deviation from a circle.
- Phase (of Moon) — shape of the Moon’s lit portion as seen from Earth.
- Perigee — Moon closest to Earth.
- Apogee — Moon farthest from Earth.
- Solar eclipse — Moon blocks Sun from Earth.
- Lunar eclipse — Earth’s shadow blocks sunlight from reaching Moon.
- Spring tide — Highest high and lowest low tides.
- Neap tide — Lowest high and highest low tides.
Action Items / Next Steps
- Review the lunar phase cycle and practice identifying each phase.
- Understand the geometry of eclipses and the orientation of Sun, Earth, and Moon.
- Complete the lunar phases assignment provided in the course module.