Earth & The Solar System

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Notes

The Earth, Moon & Sun

Earth **rotates** on its axis once every **24 hours**, causing day and night.
Earth **orbits** the Sun once every **365 days** (one year).
Earth's axis is tilted at **23.5°** from the vertical, causing the **seasons**.
The Moon is a **natural satellite** orbiting Earth every **28 days**; it rotates once per orbit so the same side always faces Earth.
Moon phases result from reflected sunlight; order: new moon → first quarter → full moon → last quarter.
Day and night are caused by Earth's rotation; seasons are caused by the combination of Earth's orbit and axial tilt.

The Solar System

The Solar System consists of the **Sun**, eight planets, dwarf planets, moons, asteroids, and comets.
Planets in order from Sun: **Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune**.
Inner planets (Mercury, Venus, Earth, Mars) are **rocky and small**; outer planets (Jupiter, Saturn, Uranus, Neptune) are **gaseous and large**.
**Asteroids** are rocky objects found mainly in the **Asteroid Belt** between Mars and Jupiter.
**Comets** are icy objects with **highly elliptical orbits**; they develop a tail when near the Sun.
**Dwarf planets** (e.g., Pluto) have weaker gravity and cannot clear their orbits of debris.

Formation of the Solar System

The Solar System formed about **4.5 billion years ago** from a **nebula** (cloud of dust and gas).
Gravity pulled the nebula together; the **Sun** formed at the centre, and an **accretion disc** of leftover matter formed around it.
In the hot inner region, only **metals and rocky materials** could solidify → **inner rocky planets**.
In the cool outer region, **light gases (H, He)** condensed → **outer gas giants**.
**Accretion** is the process where particles stick together under gravity to form larger objects.

Light Speed Calculations

Light travels at **3 × 10⁸ m/s** in a vacuum.
Time for light to travel a distance: **time = distance / speed of light**.
Light from Sun takes **8 minutes** to reach Earth, ~**3 minutes** to Mercury, ~**5 hours** to outer Solar System.
Light from the nearest star (Proxima Centauri) takes **4 years** to reach Earth.

Gravitational Field Strength

Gravitational field strength at a planet's surface depends on its **mass** and **radius**.
Greater mass → stronger gravitational field; greater distance from planet → weaker field.
The Sun contains **>99%** of the Solar System's mass, so its gravity keeps planets in orbit.
As distance from Sun increases, **orbital speed decreases** and **orbital period increases**.

Orbital Speed Equation (Extended)

For a circular orbit, orbital speed **v = 2πr / T**, where r is orbital radius and T is orbital period.
Orbital radius is measured from the **centre** of the orbited body.
Ensure units: convert km to m, minutes to seconds as needed.
Example: Earth's orbital speed ≈ **30 km/s** (from data table).

Elliptical Orbits (Extended)

Planets have **slightly elliptical** orbits; comets have **highly elliptical** orbits.
The Sun is at one **focus** of the ellipse (not the centre).
As a comet approaches the Sun, its **speed increases** (KE increases, GPE decreases).
As it moves away, its **speed decreases** (GPE increases, KE decreases); energy is conserved.

Reflection and refraction of light (not directly related to topic, but used to illustrate light speed concepts)

Practice questions

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1.Which of the planets listed is the gaseous planet which is closest to the Sun?
Easy
- AJupiter
- BMars
- CSaturn
- DUranus
2.Which of the following objects are described as orbiting the Sun? 1. Asteroids 2. Comets 3. Galaxies 4. Moons
Easy
- A1 and 2 only
- B1, 2 and 3
- C1, 2, and 4
- D1, 2, 3 and 4
3.What creates the periodic nature of the seasons?
Easy
- AThe tilting of the Earth's axis
- BThe orbiting of the Earth around the Sun
- CThe combination of the orbiting of the Earth around the Sun and the Earth's tilting axis
- DThe change in temperature of the Sun
4.Which of the following statements about the strength of a gravitational field is correct?
Easy
- AAt the surface of a planet it depends on the diameter of the planet
- BAround a planet it decreases as the distance from the planet increases
- CAround a planet it increases as the distance from the planet increases
- DAt the surface of a planet it depends on the mass of the planet
5.Which type of object orbits the Sun?
Easy
- Aan interstellar cloud of gas and dust
- Ba comet
- Ca galaxy
- Danother star
6.Which line in the table best describes the angle of the Earth’s axis from the vertical, and the effect of this tilt? | Angle of tilt | Effect of tilt | |---------------|----------------| | A 23.4° | Rising and setting of the Sun | | B 23.4° | Changing of the seasons | | C 66.6° | Rising and setting of the Sun | | D 66.6° | Changing of the seasons |
Medium
- AA
- BB
- CC
- DD
7.Which line in the table correctly identifies the length of one year, one day and daylight hours during an equinox, all as experienced on Earth? | One Earth year / days | One Earth day / hours | Daylight hours during an equinox / hours | |-----------------------|-----------------------|------------------------------------------| | A 356 | 8 | 8 | | B 364 | 12 | 12 | | C 365 | 12 | 8 | | D 365 | 24 | 12 |
Medium
- AA
- BB
- CC
- DD
8.Extended tier only Which of the following statements about orbits is true?
Medium
- AAll planets, minor planets and comets have elliptical orbits
- BThe Sun is at the centre of an elliptical orbit
- COnly comets and minor planets have elliptical orbits
- DAn object in an elliptical orbit has a constant orbital radius

Slides

The Earth, Moon & Sun

The Earth rotates on its axis once every 24 hours, causing day and night.
The Earth orbits the Sun once every 365 days.
The Earth's axis is tilted at approximately 23.5° from the vertical.

What causes the cycle of day and night on Earth?

The Earth orbiting the Sun
The Earth rotating on its axis
The Moon orbiting the Earth
The tilt of the Earth's axis

The Seasons

Seasons are caused by the Earth's orbit around the Sun and the tilt of its axis.
When the northern hemisphere is tilted towards the Sun, it receives more solar radiation and experiences summer.
When it is tilted away, it receives less radiation and experiences winter.
The tilt remains at 23.5° throughout the orbit.

Which of these are true about the seasons?

Seasons are caused by the Earth's distance from the Sun.
The Earth's axis is tilted at 23.5°.
When it is summer in the northern hemisphere, it is winter in the southern hemisphere.
The tilt of the Earth's axis changes throughout the year.

The Moon

The Moon is a natural satellite that orbits Earth in about 28 days.
It rotates on its axis once every 28 days, so the same side always faces Earth.
The Moon does not produce its own light; it reflects sunlight.
As it orbits, we see different phases (new moon, first quarter, full moon, last quarter).

How long does it take for the Moon to complete one orbit of Earth?

24 hours
28 days
365 days
7 days

The Solar System

The Solar System consists of the Sun, eight planets, dwarf planets, moons, asteroids, and comets.
The planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Inner planets (Mercury, Venus, Earth, Mars) are rocky and small.
Outer planets (Jupiter, Saturn, Uranus, Neptune) are gaseous and large.

The planet closest to the Sun is ____.

Formation of the Solar System

The Solar System formed from a nebula (cloud of dust and gas) about 4.5 billion years ago.
Gravity pulled the nebula together, forming the Sun at the centre.
An accretion disc formed around the Sun, where planets formed.
In the hot inner region, only heavy elements (metals) could solidify, forming rocky planets.
In the cooler outer region, light gases (hydrogen, helium) condensed, forming gas giants.

Why are the inner planets rocky and small?

They formed from light gases.
The high temperature near the Sun allowed only heavy elements to solidify.
They are made of ice.
They formed from the accretion disc.

Light Speed Calculations

Light travels at a constant speed of 3.0 × 10⁸ m/s.
Time = distance / speed of light.
Example: Light from the Sun takes about 8 minutes to reach Earth.

The speed of light is 3.0 × 10⁸ ____.

Gravitational Field Strength

Gravitational field strength depends on the mass of the planet and distance from it.
Greater mass → stronger gravitational field.
As distance from a planet increases, gravitational field strength decreases.
The Sun's gravity keeps planets in orbit.

What happens to the gravitational field strength as you move away from a planet?

It increases.
It decreases.
It stays the same.
It depends on the planet's colour.

Orbital Speed Equation (Extended)

For a circular orbit, orbital speed v = 2πr / T.
r is the orbital radius (from centre of orbited body).
T is the orbital period (time for one orbit).
Example: Earth's orbital speed is about 30 km/s.

The orbital speed equation is v = 2πr / ____.

Elliptical Orbits (Extended)

Planets have slightly elliptical orbits; comets have highly elliptical orbits.
The Sun is at one focus of the ellipse.
As a comet approaches the Sun, its speed increases (kinetic energy increases).
As it moves away, its speed decreases (gravitational potential energy increases).
Energy is conserved: gravitational potential energy ↔ kinetic energy.

Which of these are true about comets?

Comets have highly elliptical orbits.
Comets travel fastest when farthest from the Sun.
As a comet approaches the Sun, its kinetic energy increases.
The Sun is at the centre of a comet's orbit.

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