Rotation: Earth Spins on Its Axis
Rotation is the spinning of Earth on its axis, completing one full turn every 24 hours.
The rate of rotation of the pendulum's plane depends on latitude. At 40° N, it takes about 37 hours for a full rotation.
Coriolis Effect
- The Coriolis effect describes how moving objects, like winds and ocean currents, follow curved paths due to Earth's rotation.
Northern Hemisphere: Objects deflect to the right.
Southern Hemisphere: Objects deflect to the left.
Don't confuse rotation with revolution. Rotation refers to Earth spinning on its axis, while revolution describes Earth's orbit around the Sun.
Revolution: Earth's Orbit Around the Sun
Revolution is the movement of Earth around the Sun, completing one orbit in approximately 365.25 days.
Evidence of Earth's Revolution
Annual Traverse of Constellations
- As Earth orbits the Sun, different constellations become visible at night.
- This cyclical pattern provides evidence of Earth's changing position in space.
* Orionis visible in winter, while Scorpius appears in summer.
Parallelism of Earth's Axis
- Earth's axis remains tilted at 23.5° and points toward Polaris throughout its orbit.
- This tilt causes seasonal changes as different hemispheres receive varying amounts of sunlight.
Earth's axis is not perpendicular to its orbital plane. This tilt is responsible for the changing seasons.
Precession: The Wobble of Earth's Axis
Precession is the slow, conical movement of Earth's axis, completing one cycle every 26,000 years.
- Caused by gravitational pulls from the Sun and Moon.
- Over time, precession alters the position of the North Star. For example, in 13,000 years, Vega will replace Polaris as the North Star.
Precession has minimal impact on daily life but affects long-term astronomical observations and the timing of seasons over millennia.
Effects of Earth's Motions
1. The Sun's Apparent Path
- The Sun appears to move across the sky due to Earth's rotation.
- Its path varies with seasons and latitude.
* In New York City, the Sun's altitude at noon is about 73° on June 21 but only 23° on December 21.
On March 21, the Sun is directly overhead at the equator (0°). In New York City (41° N), the Sun's altitude is \$90° - 41° = 49°\$.
The Earth-Moon-Sun System
1. Phases of the Moon
- The Moon reflects sunlight, and its phases depend on its position relative to Earth and the Sun.
- A complete cycle of phases takes 29.5 days, slightly longer than the Moon's orbital period of 27.32 days.
The extra 2 days are needed for the Moon to realign with the Sun due to Earth's movement in its orbit.
2. Eclipses
- Solar Eclipse: Occurs when the Moon passes between Earth and the Sun, casting a shadow on Earth.
- Total Eclipse: Observed in the umbra (full shadow).
- Partial Eclipse: Observed in the penumbra (partial shadow).
- Lunar Eclipse: Occurs when the Moon passes through Earth's shadow
- Total Eclipse: The Moon appears red due to sunlight refracted by Earth's atmosphere.
- Partial Eclipse: Only part of the Moon enters Earth's shadow.
Eclipses do not occur every month because the Moon's orbit is tilted about 5° relative to Earth's orbit.
3. Tides
- Caused by: Gravitational pull of the Moon and Sun.
- Tidal Bulges: Form on the side of Earth facing the Moon and the opposite side.
- Types of Tides:
- Spring Tides: Higher tides during new and full moons when the Sun and Moon align.
- Neap Tides: Lower tides during first and third-quarter moons when the Sun and Moon are at right angles.
The Moon's gravity has a greater effect on tides than the Sun's because it is closer to Earth, despite the Sun's larger mass.
Reflection and Broader Connections
- Earth's rotation and revolution explain day and night, seasons, and the apparent motion of celestial objects.
- These motions are fundamental to understanding natural phenomena like tides, eclipses, and the phases of the Moon.
