Overview
This lecture covers the interactions between Earth, the Sun, and the Moon, and how these relationships cause phenomena such as day/night, seasons, eclipses, lunar phases, and tides.
The Earth-Sun-Moon System
- The Moon revolves around Earth, which revolves around the Sun.
- Earth completes one revolution around the Sun in about 365.25 days (one year).
- Earth's orbit is an ellipse, with the Sun at one focus.
- The solar system consists of the Sun, planets, moons, asteroids, and other celestial bodies bound by gravity.
Earth’s Rotation and Time
- Earth rotates 360° on its axis every 24 hours (15° per hour), creating day and night.
- This rotation is responsible for time zones (24 globally); east is later than west.
- The apparent motion of the Sun and stars rising in the east and setting in the west is an illusion caused by Earth’s rotation.
- Foucault's pendulum provides evidence of Earth's rotation.
Seasons and Sun Path
- Earth’s axis is tilted, causing seasonal changes as different hemispheres tilt toward or away from the Sun.
- Summer: hemisphere tilted toward the Sun (direct rays, longer days).
- Winter: hemisphere tilted away from the Sun (less sunlight, shorter days).
- The Sun’s apparent path in the sky is highest in summer and lowest in winter.
Moon Phases and Eclipses
- Moon phases result from varying amounts of the Sun’s light seen from Earth as the Moon orbits us.
- Waxing: increasing illumination; waning: decreasing illumination.
- The full lunar cycle includes new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent.
- A solar eclipse occurs when the Moon is between the Sun and Earth; a lunar eclipse occurs when Earth is between the Sun and the Moon.
- Eclipses only happen when the Sun, Earth, and Moon are aligned.
Tides
- Tides are caused primarily by the Moon’s gravity, with some influence from the Sun.
- Tidal force creates two bulges of water (high tides) on opposite sides of Earth.
- Most places experience two high tides and two low tides daily.
Key Terms & Definitions
- Celestial body — natural object in space, e.g., the Sun, planets, moons.
- Orbit (Ellipse) — the path an object takes around another; elliptical means oval-shaped.
- Time zone — regions of Earth with the same standard time, based on Earth's rotation.
- Foucault's pendulum — device demonstrating Earth's rotation.
- Solstice — time when the Sun’s path is farthest north or south from the equator.
- Equinox — day and night are approximately equal in length.
- Phases of the Moon — shapes of illuminated Moon seen from Earth.
- Eclipse — event where one celestial body moves into the shadow of another.
- Umbra/Penumbra — darkest/partial shadow cast during eclipses.
- Tidal force — difference in gravity's pull, causing Earth's water to bulge.
Action Items / Next Steps
- Review the phases of the Moon and understand the differences between waxing and waning.
- Prepare for an upcoming episode focused entirely on gravity.
- Observe local tide schedules and relate them to the position of the Moon.