The Earth and the Solar System

6.1.1 The Earth

Earth's Rotation and the Day-Night Cycle

The Earth is a planet (a large celestial body that orbits a star). It has two main types of motion:

1. Rotation - The Earth spins around on its own axis
2. Revolution/Orbit - The Earth travels around the Sun

Earth's Axis

The axis is an imaginary line that passes through the North and South Poles of the Earth. Think of it like a rod going straight through the Earth from top to bottom.

• The Earth's axis is tilted at an angle of approximately 23.5° from vertical
• This tilt stays the same throughout the year as Earth orbits the Sun
• The Earth rotates (spins) on its axis once every 24 hours

Day and Night

The Earth's rotation on its axis causes the cycle of day and night:

• As the Earth spins, only half of its surface faces the Sun at any time
• The half facing the Sun experiences daytime (it receives sunlight)
• The half facing away from the Sun experiences nighttime (it's in shadow)
• It takes 24 hours for any point on Earth to complete one full rotation and return to the same position

The Apparent Motion of the Sun

Even though the Sun doesn't actually move around the Earth, it appears to move across our sky. This is called the apparent motion of the Sun, and it's caused by Earth's rotation.

Each day, the Sun appears to:

• Rise from the east in the morning
• Move across the sky during the day
• Reach its highest point at noon (12 pm)
• Set in the west in the evening

This happens because as the Earth rotates from west to east, we see the Sun appearing to move from east to west.

Earth's Orbit and the Seasons

Earth's Orbit Around the Sun

The Earth travels around the Sun in a path called an orbit. This orbit is slightly elliptical (oval-shaped), but for most purposes we can treat it as circular.

• The Earth completes one full orbit around the Sun in approximately 365 days (one year)
• The distance from the Earth to the Sun is approximately 150 million km

What Causes the Seasons?

The seasons (summer, autumn, winter, spring) are caused by two factors working together:

1. The tilt of Earth's axis (23.5°)
2. The Earth's orbit around the Sun

As the Earth orbits the Sun, the tilt of its axis stays pointing in the same direction. This means that at different times of year, different parts of Earth are tilted toward or away from the Sun.

How the Tilt Creates Seasons

When one hemisphere (half of Earth) tilts toward the Sun:

• That hemisphere receives more direct sunlight
• The Sun's rays hit the surface at a steeper angle
• Days are longer than nights
• Temperatures are warmer
• This hemisphere experiences summer

When the same hemisphere tilts away from the Sun:

• That hemisphere receives less direct sunlight
• The Sun's rays hit the surface at a shallower angle (more spread out)
• Days are shorter than nights
• Temperatures are cooler
• This hemisphere experiences winter

Important: When it's summer in the Northern Hemisphere (the top half of Earth), it's winter in the Southern Hemisphere (the bottom half), and vice versa.

Equinoxes and Solstices

These are special days that mark changes in the seasons:

Equinoxes (occur twice per year):

• Around March 20th (Spring/Vernal Equinox in Northern Hemisphere)
• Around September 23rd (Autumn Equinox in Northern Hemisphere)
• On these days, the Sun is directly above the Equator
• Day and night are approximately equal in length everywhere on Earth
• The Sun rises exactly in the east and sets exactly in the west

Solstices (occur twice per year):

• Around June 21st (Summer Solstice in Northern Hemisphere)

  • Longest day and shortest night in Northern Hemisphere
  • Shortest day and longest night in Southern Hemisphere
  • Sun rises in the northeast and sets in the northwest (Northern Hemisphere)

• Around December 21st (Winter Solstice in Northern Hemisphere)

  • Shortest day and longest night in Northern Hemisphere
  • Longest day and shortest night in Southern Hemisphere
  • Sun rises in the southeast and sets in the southwest (Northern Hemisphere)

Common Misconception: Seasons are NOT caused by Earth being closer to or farther from the Sun. The distance changes slightly due to Earth's elliptical orbit, but this has minimal effect. The tilt is what matters!

The Moon and Its Phases

The Moon's Orbit

The Moon is Earth's natural satellite (an object that orbits a planet).

• The Moon orbits around the Earth in approximately 28 days (about one month)
• The Moon also rotates on its own axis once every 28 days
• Because the Moon's rotation and orbit take the same time, the same side of the Moon always faces Earth

Phases of the Moon

The Moon doesn't produce its own light - it reflects sunlight. As the Moon orbits Earth, we see different amounts of the Moon's lit surface, creating the phases.

Key phases in order:

1. New Moon (Day 0):

   • Moon is between Earth and the Sun
   • The side facing Earth is in darkness
   • We cannot see the Moon in the sky

2. Waxing Crescent (Days 1-6):

   • A thin crescent appears and grows (waxes means "grows")

3. First Quarter (Day 7):

   • Half of the Moon's surface facing Earth is illuminated
   • Called "quarter" because the Moon has completed ¼ of its orbit

4. Waxing Gibbous (Days 8-13):

   • More than half but not all of the Moon is visible
   • Continues to grow (wax)

5. Full Moon (Day 14):

   • Earth is between the Moon and the Sun
   • The entire side facing Earth is illuminated
   • We see the Moon as a complete circle

6. Waning Gibbous (Days 15-20):

   • The Moon starts to shrink (wane means "shrink")
   • Still more than half visible

7. Last/Third Quarter (Day 21):

   • Half of the Moon's surface facing Earth is illuminated
   • The Moon has completed ¾ of its orbit

8. Waning Crescent (Days 22-28):

   • A thin crescent that continues to shrink
   • Soon returns to New Moon (Day 29)

Remember: Exactly half of the Moon is always illuminated by the Sun, but how much of that illuminated half we can see from Earth changes as the Moon orbits.

Average Orbital Speed (Supplement)

What is Orbital Speed?

Orbital speed is the average speed at which an object (like a planet or moon) travels around another object in its orbit.

The Equation

The average orbital speed can be calculated using:

v = 2πr / T

Where:

• v = orbital speed in metres per second (m/s)
• r = average radius of the orbit in metres (m) - this is the distance from the centre of the object being orbited to the orbiting object
• T = orbital period in seconds (s) - this is the time taken to complete one full orbit
• 2πr = the circumference (distance around) of a circular orbit

Why "average" orbital speed?

Most orbits are slightly elliptical (oval-shaped), so the actual speed varies slightly. However, we use the average radius and calculate an average speed.

Example Calculation:

Calculate the orbital speed of Earth as it orbits the Sun.

• Distance from Sun (orbital radius): r = 1.5 × 10¹¹ m
• Orbital period: T = 365 days = 365 × 24 × 60 × 60 = 31,536,000 s

Step 1: Write the equation v = 2πr / T

Step 2: Substitute the values v = (2 × π × 1.5 × 10¹¹) / 31,536,000

Step 3: Calculate v = 29,900 m/s (approximately 30,000 m/s or 30 km/s)

Important tips:

• Always convert the time period to seconds
• Always convert distances to metres
• The radius r must be measured from the centre of the object being orbited, not from its surface