Cheatsheet: Earth in Space

1. Earth's Place in the Solar System

1.1 The Solar System Structure

1.2 Earth's Position and Characteristics

• Third planet from the Sun
• Distance from Sun: 150 million km (93 million miles) - 1 Astronomical Unit (AU)
• Only known planet with liquid water on surface
• Atmosphere contains oxygen and nitrogen
• One natural satellite: the Moon
• Diameter: 12,742 km

2. Earth's Rotation and Revolution

2.1 Earth's Rotation

2.2 Earth's Revolution

2.3 Why We Have Leap Years

• Earth takes 365.25 days to orbit the Sun, not exactly 365 days
• Extra 0.25 day accumulates to one full day every 4 years
• Leap year adds February 29th every 4 years to keep calendar aligned

3. Seasons

3.1 Cause of Seasons

• Seasons are caused by Earth's 23.5-degree axial tilt, NOT by distance from Sun
• Tilt causes different amounts of sunlight to reach different hemispheres during the year
• When Northern Hemisphere tilts toward Sun, it experiences summer; Southern Hemisphere experiences winter
• When Southern Hemisphere tilts toward Sun, it experiences summer; Northern Hemisphere experiences winter

3.2 Key Seasonal Positions

3.3 Seasonal Effects

• Sun's angle determines intensity of heating: more direct rays = more heating
• Length of daylight varies with season
• Seasons are opposite in Northern and Southern Hemispheres
• Equator experiences minimal seasonal change due to consistent Sun angle

4. The Moon

4.1 Moon Properties

4.2 Moon Phases

4.3 Moon Phase Cycle

• Complete cycle takes 29.5 days (lunar month)
• Phases result from changing positions of Moon relative to Earth and Sun
• Moon reflects sunlight; does not produce its own light
• Same side of Moon always faces Earth due to synchronous rotation

5. Eclipses

5.1 Solar Eclipse

5.2 Lunar Eclipse

5.3 Why Eclipses Don't Happen Every Month

• Moon's orbit is tilted 5 degrees relative to Earth's orbit around Sun
• Moon usually passes above or below Earth's shadow (lunar eclipse) or Moon's shadow misses Earth (solar eclipse)
• Eclipses only occur when Moon crosses the plane of Earth's orbit at new or full moon

6. Tides

6.1 Causes of Tides

• Gravitational pull of Moon on Earth's oceans is primary cause
• Sun also contributes to tides but has less effect than Moon (Sun is farther away)
• Moon's gravity pulls water toward it, creating bulge on side facing Moon
• Inertia creates second bulge on opposite side of Earth
• Earth rotates through these bulges, causing two high tides and two low tides per day

6.2 Types of Tides

7. Stars and Constellations

7.1 Stars

7.2 Constellations

• Constellation: pattern of stars as seen from Earth; 88 official constellations
• Stars in constellation are not close together in space; they only appear grouped from Earth's viewpoint
• Different constellations visible in different seasons as Earth orbits Sun
• Circumpolar constellations: visible year-round in Northern or Southern Hemisphere; circle around celestial pole

7.3 Common Constellations

8. Gravity in Space

8.1 Universal Gravitation

• Gravity is force of attraction between all objects with mass
• Greater mass = stronger gravitational pull
• Closer distance = stronger gravitational pull
• Gravity holds planets in orbit around Sun
• Gravity holds Moon in orbit around Earth
• Gravity keeps atmosphere and objects on Earth's surface

8.2 Orbits

9. Models of the Solar System

9.1 Historical Models

9.2 Evidence for Heliocentric Model

• Galileo observed moons orbiting Jupiter, proving not everything orbits Earth
• Venus shows complete cycle of phases, only possible if it orbits Sun
• Retrograde motion of planets explained by different orbital speeds
• Mathematical calculations match observed planetary positions