Call Now! :+254 726 126 859 | +254 739 289 008
Earth & the Solar System
TOPIC 24: EARTH AND THE SOLAR SYSTEM
6.1.1 The Earth
Earth's Orbit
- Earth orbits the Sun in a slightly elliptical path (approximated as circular for calculations)
- Orbital speed: approximately 30 km/s
- Orbital period: 365 days (1 year)
Seasons
Important: Seasons are caused by Earth's tilted axis, NOT by distance from the Sun!
- When Northern Hemisphere tilts toward Sun → Spring/Summer (longer days)
- When Northern Hemisphere tilts away from Sun → Autumn/Winter (shorter days)
- Southern Hemisphere experiences opposite seasons simultaneously
Key Points in Earth's Orbit
- Summer Solstice (Point C): Longest day in Northern Hemisphere, shortest in Southern
- Winter Solstice (Point G): Shortest day in Northern Hemisphere, longest in Southern
- Equinoxes (Points A & D): Day and night equal length everywhere
Poles: Experience 24 hours of darkness (winter) or 24 hours of light (summer) due to Earth's tilt
The Moon
- Shines by reflecting sunlight (produces no light of its own)
- Takes same time to orbit Earth as to rotate on its axis
- Shows periodic cycle of phases as it orbits
Tides
- Spring tides: Unusually high/low tides when Sun, Moon, and Earth are aligned
- Neap tides: Weaker tides when Sun and Moon are at right angles to Earth
6.1.2 The Solar System
Structure
- Sun at center (star, over 99% of solar system mass)
- 8 planets orbit the Sun
- Satellites: Natural (moons) or Artificial (e.g., ISS)
The Sun
- Medium-sized star
- Composed mostly of hydrogen and helium
- Mass: 2×10³⁰ kg (1 solar mass)
- Radiates energy in infrared, visible, and ultraviolet regions
Orbital Mechanics
Key Principle: Planets in elliptical orbits follow these rules:
- Closer to Sun → Stronger gravitational force → Faster orbital speed
- Farther from Sun → Weaker gravitational force → Slower orbital speed
Conservation of energy: Objects trade GPE for KE (faster when closer, slower when farther)
Planet Data (Inner to Outer)
| Planet | Distance (million km) | Orbital Speed (km/s) | Period |
|---|---|---|---|
| Mercury | 57.9 | 47.9 | 88 days |
| Venus | 108.2 | 35.0 | 225 days |
| Earth | 149.6 | 29.8 | 365 days |
| Mars | 227.9 | 24.1 | 687 days |
| Jupiter | 778.6 | 13.1 | 11.9 years |
| Saturn | 1433.5 | 9.7 | 29.5 years |
| Uranus | 2872.5 | 6.8 | 75 years |
| Neptune | 4495.1 | 5.4 | 165 years |
Other Solar System Objects
- Asteroids: Small rocky objects, most in asteroid belt between Mars and Jupiter
- Comets: Made of dust and ice, highly elliptical orbits, ice melts near Sun forming tail
Important Distances & Times
Speed of light: c = 3.0 × 10⁸ m/s
Light from Sun to Earth: approximately 500 seconds (8 min 20 sec)
Distance Sun to Earth: approximately 150 million km (1.5 × 10¹¹ m)
Light-year: Distance light travels in one year = 9.5 × 10¹⁵ m
Formation of Solar System (Accretion Model)
- Solar nebula (gas and dust cloud) collapsed under its own gravity
- Gravity caused particles to form spinning accretion disc
- Heavier particles moved to center, heat/pressure triggered nuclear fusion → Sun formed
- Dust particles combined → inner rocky planets
- Lighter gases stayed farther out → gas giants
Gravitational Field Strength
Definition: Force per unit mass experienced by object in gravitational field
- Depends on mass of planet and distance from planet
- Keeps objects in orbit around Sun
Here you go — all square brackets **and everything inside them** removed, with the rest kept exactly the same.
---
Definitions
- Orbital Speed: The speed required for an object to remain in orbit around another celestial body.
- Satellite: An object that moves around a larger object. These include natural satellites (e.g., moons) and artificial satellites (e.g., the International Space Station).
- Gravitational Field Strength: The force per unit mass experienced by an object within a gravitational field.
- Asteroid: A small rocky object that orbits the Sun, with most located in the asteroid belt between Mars and Jupiter.
- Comet: An object made of dust and ice that orbits the Sun in a highly elliptical path.
- Light-year: The distance light travels in a vacuum over the course of one year.
- Equinox: Occurs when day and night are of equal length (occurs at points A and D in Earth's orbit).
- Solstice: Occurs when the tilt of the Earth's axis causes the longest (Summer) or shortest (Winter) day of the year.
Formulae and Key Constants
- Relationship between Force and Speed: F = ma = (m × Δv) / t. This shows that force (F) is proportional to velocity (v); therefore, a decrease in gravitational force leads to a decreased speed of revolution.
- Speed of Light (c): Approximately 3.0 × 10^8 m/s.
- Distance Calculation: Distance = Speed × Time.
- One Light-Year: Approximately 9.5 × 10^15 meters.
- Solar Mass: The mass of the Sun is 2 × 10^30 kg.
Summary Notes
The Earth
- Rotation: The Earth rotates on its tilted axis approximately every 24 hours, causing day, night, and the apparent daily motion of the Sun.
- Orbit: The Earth orbits the Sun roughly every 365 days in a slightly elliptical path.
- Seasons: Caused by the Earth’s 23.5° axial tilt. When a hemisphere is tilted toward the Sun, it receives stronger sunlight (Summer); when tilted away, it receives less (Winter).
- The Moon: Reflects sunlight and does not produce its own. Its rotation and revolution periods are the same, leading to a periodic cycle of phases as seen from Earth.
The Solar System
- The Sun: A medium-sized star made mostly of hydrogen and helium, containing over 99% of the solar system's mass.
- Gravity and Distance: Gravitational force decreases as distance from the Sun increases, which causes the orbital speed of farther planets to be slower.
- Accretion Model: The solar system formed from a collapsing solar nebula. Gravity created a spinning accretion disc where heavier particles formed rocky inner planets and lighter gases formed outer gas giants.
- Energy Conservation: In elliptical orbits, objects trade gravitational potential energy (GPE) for kinetic energy (KE). They move faster when closer to the Sun as GPE is converted to KE.
Why the Moon shows the same face to Earth
How to read the animation
- The Moon (small grey circle) revolves around Earth along the dashed orbit.
- The small dark mark on the Moon is a fixed feature on the Moon’s surface (a “face” marker).
- The Moon group both revolves around Earth and simultaneously rotates about its own center.
Key point: in the animation the Moon completes exactly one rotation while it completes one full orbit. That is why the same face always points to Earth—rotation period = orbital period.
Tip: use the controls below to speed up or slow the motion to observe the match.
12s
Abel.Masitsa.com Works!
🔑 The Relationship Made Simple
For the same face of the Moon to always face Earth:
The Moon must rotate exactly once during one full orbit around Earth.
If:
Rotation was faster → we would see different sides
Rotation was slower → we would also see different sides
Only 1 rotation per orbit = same face always facing us.
🧲 But WHY did it become like this?
Because of tidal locking:
Long ago, the Moon rotated faster.
Earth’s gravity pulled on the Moon’s uneven mass (the “bulge”).
This produced friction that slowed the Moon’s rotation.
It slowed until rotation matched orbit → then friction stopped.
This is the “balanced” state, so it remains locked.
📌 Short Version
The Moon rotates once in the same time it orbits once.
That is why the same side faces Earth always.
Gravity caused this synchronization → tidal locking.